diff --git a/scripts/classroom-reboot-when-idle b/scripts/classroom-reboot-when-idle
new file mode 100755
index 0000000..c7a81c4
--- /dev/null
+++ b/scripts/classroom-reboot-when-idle
@@ -0,0 +1,157 @@
+#!/usr/bin/env python3
+"""Reboot after an update, but only once nobody is using the computer.
+
+The nightly update check (ekklesia-classroom-update.timer, 03:00, and
+Persistent so that a computer switched off at 03:00 checks when it is next
+switched on) used to reboot the moment an update was installed. At 03:00
+that is harmless. Switched on in the morning after a night off, it meant a
+classroom computer restarting itself half an hour into the day, in front
+of a teacher.
+
+So the updater installs, then starts this, and this reboots only when:
+
+    nothing has touched any keyboard, mouse, touchpad or touchscreen for
+    IDLE_SECONDS — seen here as kernel input events, which is what every
+    one of them produces, whatever the desktop is doing; and
+
+    no sound has played for as long — a video to the room has no input for
+    an hour and is not an idle computer, and the end of it is not the
+    moment to restart either. Seen as the kernel's own playback state
+    (/proc/asound/.../status says RUNNING while a stream plays).
+
+At 03:00 that is about 03:30. In the middle of a morning it is half an
+hour after the teacher last touched it. If the computer is switched off
+first, nothing is lost: the update is installed, and the next start is on
+it. Until then the running session carries on as it was.
+
+The time starts when this starts: input from before cannot be seen, so a
+computer is only called idle after this has watched it be idle.
+
+    classroom-reboot-when-idle [--idle-seconds N] [--reboot-command CMD...]
+
+The options are for the lab; the service runs it with none.
+"""
+
+import argparse
+import glob
+import os
+import select
+import subprocess
+import sys
+import time
+
+IDLE_SECONDS = 30 * 60
+
+# How often it looks: input is noticed at once, the rest once a minute is
+# plenty for a half-hour decision.
+CHECK_SECONDS = 60
+
+INPUT_GLOB = "/dev/input/event*"
+ASOUND_ROOT = "/proc/asound"
+
+
+def log(message):
+    print(f"[classroom-reboot-when-idle] {message}", flush=True)
+
+
+def audio_playing(root=ASOUND_ROOT):
+    """Is any sound card playing right now?"""
+    for status in glob.glob(os.path.join(root, "card*", "pcm*p", "sub*", "status")):
+        try:
+            with open(status, encoding="utf-8", errors="replace") as handle:
+                if handle.readline().strip() == "state: RUNNING":
+                    return True
+        except OSError:
+            continue
+
+    return False
+
+
+def should_reboot(now, last_input, idle_seconds, playing):
+    """The decision: long enough without input, and nothing playing."""
+    return not playing and now - last_input >= idle_seconds
+
+
+class Inputs:
+    """Every input device the kernel has, reopened as they come and go."""
+
+    def __init__(self, pattern=INPUT_GLOB):
+        self.pattern = pattern
+        self.open = {}
+
+    def rescan(self):
+        present = set(glob.glob(self.pattern))
+
+        for fd, path in list(self.open.items()):
+            if path not in present:
+                os.close(fd)
+                del self.open[fd]
+
+        known = set(self.open.values())
+
+        for path in sorted(present - known):
+            try:
+                self.open[os.open(path, os.O_RDONLY | os.O_NONBLOCK)] = path
+            except OSError:
+                continue
+
+    def wait(self, seconds):
+        """True if anything happened on any device within SECONDS."""
+        if not self.open:
+            time.sleep(seconds)
+            return False
+
+        ready, _, _ = select.select(list(self.open), [], [], seconds)
+        seen = False
+
+        for fd in ready:
+            try:
+                while os.read(fd, 4096):
+                    seen = True
+            except BlockingIOError:
+                pass
+            except OSError:
+                # A device that went away mid-read; the next rescan drops it.
+                pass
+
+        return seen
+
+
+def main(argv=None):
+    parser = argparse.ArgumentParser(description=__doc__.splitlines()[0])
+    parser.add_argument("--idle-seconds", type=float, default=IDLE_SECONDS)
+    parser.add_argument("--check-seconds", type=float, default=CHECK_SECONDS)
+    parser.add_argument("--input-glob", default=INPUT_GLOB)
+    parser.add_argument("--asound-root", default=ASOUND_ROOT)
+    parser.add_argument("--reboot-command", nargs="+", default=["systemctl", "reboot"])
+    args = parser.parse_args(argv)
+
+    inputs = Inputs(args.input_glob)
+    inputs.rescan()
+    last_input = time.monotonic()
+    log(f"an update is installed; rebooting once nobody has used this computer for "
+        f"{round(args.idle_seconds / 60)} minutes and nothing is playing "
+        f"(watching {len(inputs.open)} input devices)")
+
+    while True:
+        deadline = time.monotonic() + args.check_seconds
+
+        while time.monotonic() < deadline:
+            if inputs.wait(max(0.0, min(5.0, deadline - time.monotonic()))):
+                last_input = time.monotonic()
+
+        inputs.rescan()
+        playing = audio_playing(args.asound_root)
+
+        if playing:
+            # Sound counts as someone there: a 40-minute video ending is
+            # not the moment to restart, so the half hour starts after it.
+            last_input = time.monotonic()
+
+        if should_reboot(time.monotonic(), last_input, args.idle_seconds, playing):
+            log("nobody has used it, and nothing is playing: rebooting")
+            return subprocess.run(args.reboot_command).returncode
+
+
+if __name__ == "__main__":
+    sys.exit(main())
diff --git a/scripts/ekklesia-classroom-update b/scripts/ekklesia-classroom-update
index cb18a9b..711bdb8 100755
--- a/scripts/ekklesia-classroom-update
+++ b/scripts/ekklesia-classroom-update
@@ -14,6 +14,7 @@ from urllib.parse import urljoin
 
 CONFIG = "/etc/ekklesia-classroom/device.conf"
 LOCKFILE = "/run/ekklesia-classroom-update.lock"
+REBOOT_WHEN_IDLE = "ekklesia-classroom-reboot-when-idle.service"
 CACHE_DIR = "/var/cache/ekklesia-classroom"
 PACKAGE = "ekklesia-classroom"
 
@@ -291,8 +292,12 @@ def main():
             if args.no_reboot:
                 log("Update requires reboot, but --no-reboot was specified.")
             else:
-                log("Update requires reboot. Rebooting now.")
-                subprocess.run(["systemctl", "reboot"])
+                # Not now. At 03:00 nobody is there; after a missed 03:00,
+                # checked when the computer is next switched on, somebody
+                # usually is. classroom-reboot-when-idle waits until nobody
+                # has touched it for half an hour and nothing is playing.
+                log("Update requires reboot. Rebooting once nobody is using this computer.")
+                subprocess.run(["systemctl", "start", "--no-block", REBOOT_WHEN_IDLE])
 
         return 0
 
diff --git a/systemd/updater/ekklesia-classroom-reboot-when-idle.service b/systemd/updater/ekklesia-classroom-reboot-when-idle.service
new file mode 100644
index 0000000..3c4a28b
--- /dev/null
+++ b/systemd/updater/ekklesia-classroom-reboot-when-idle.service
@@ -0,0 +1,10 @@
+[Unit]
+Description=Reboot after an Ekklesia Classroom update, once nobody is using the computer
+Documentation=file:///usr/local/bin/classroom-reboot-when-idle
+# Started by ekklesia-classroom-update after it installs an update that
+# needs a reboot; never at boot. A computer switched off before it
+# reboots simply starts on the new version.
+
+[Service]
+Type=simple
+ExecStart=/usr/local/bin/classroom-reboot-when-idle
diff --git a/tests/test-classroom-update-schedule.sh b/tests/test-classroom-update-schedule.sh
new file mode 100755
index 0000000..f96e3c6
--- /dev/null
+++ b/tests/test-classroom-update-schedule.sh
@@ -0,0 +1,182 @@
+#!/bin/bash
+# When a classroom checks for updates, and when it reboots after one.
+#
+# The check: daily at 03:00, caught up once after a computer that was off
+# at 03:00 is switched on. The reboot: only once nobody has used the
+# computer for half an hour and nothing is playing. The timer itself is
+# measured in the lab (tools/lab/update-schedule-check); here are its
+# settings, and classroom-reboot-when-idle run for real with a pipe
+# standing in for a keyboard and files standing in for sound cards.
+set -uo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+WORK="$(mktemp -d)"
+trap 'rm -rf "$WORK"' EXIT
+
+WORK="$WORK" python3 - "$ROOT" <<'PY'
+import configparser
+import importlib.machinery
+import importlib.util
+import os
+import sys
+import threading
+import time
+from pathlib import Path
+
+root = Path(sys.argv[1])
+work = Path(os.environ["WORK"])
+passed = failed = 0
+
+
+def check(label, condition, detail=""):
+    global passed, failed
+    if condition:
+        print(f"  ok   {label}")
+        passed += 1
+    else:
+        print(f"  FAIL {label}")
+        if detail:
+            print(f"       {detail}")
+        failed += 1
+
+
+loader = importlib.machinery.SourceFileLoader("idle", str(root / "scripts/classroom-reboot-when-idle"))
+spec = importlib.util.spec_from_loader("idle", loader)
+idle = importlib.util.module_from_spec(spec)
+loader.exec_module(idle)
+
+
+def unit(path):
+    parser = configparser.ConfigParser(strict=False, interpolation=None)
+    parser.optionxform = str
+    parser.read(path)
+    return parser
+
+
+print("1. the check: daily at 03:00, caught up once after being off")
+timer = unit(root / "systemd/updater/ekklesia-classroom-update.timer")
+check("every day at 03:00", timer["Timer"].get("OnCalendar") == "*-*-* 03:00:00", dict(timer["Timer"]))
+check("a computer off at 03:00 checks when it is next on (Persistent)", timer["Timer"].get("Persistent") == "true")
+check("spread over half an hour, so rooms do not all ask at once", timer["Timer"].get("RandomizedDelaySec") == "30m")
+check("and nothing else starts it: no boot or repeating trigger to loop on",
+      not any(k in timer["Timer"] for k in ("OnBootSec", "OnStartupSec", "OnUnitActiveSec", "OnUnitInactiveSec", "OnActiveSec")))
+check("enabled by the package on every install and upgrade",
+      "systemctl enable ekklesia-classroom-update.timer" in (root / "packaging/debian/postinst").read_text())
+check("and installed as a package file, so an upgrade always brings the packaged schedule",
+      "systemd/updater/ekklesia-classroom-update.timer" in (root / "tools/build-release.sh").read_text()
+      and not (root / "packaging/debian/conffiles").exists())
+
+print()
+print("2. the updater: the same checks, and no reboot of its own")
+updater = (root / "scripts/ekklesia-classroom-update").read_text()
+code = "\n".join(l for l in updater.splitlines() if not l.lstrip().startswith("#"))
+check("only the device's own channel", 'f"{channel}/manifest.json"' in code and 'channel = config["CHANNEL"]' in code)
+check("only a newer version", "version_is_newer(candidate, current)" in code)
+check("only a manifest that makes sense", "manifest_problem(manifest)" in code)
+check("only a file with the manifest's SHA-256", "actual_hash = sha256(temporary_path)" in code)
+check("only the package and version the manifest names", "if name != PACKAGE or version != candidate:" in code)
+check("never reboots by itself any more", '["systemctl", "reboot"]' not in code)
+check("but hands the reboot to classroom-reboot-when-idle, without waiting for it",
+      '"systemctl", "start", "--no-block", REBOOT_WHEN_IDLE' in code
+      and 'REBOOT_WHEN_IDLE = "ekklesia-classroom-reboot-when-idle.service"' in code)
+service = unit(root / "systemd/updater/ekklesia-classroom-reboot-when-idle.service")
+check("which is never started at boot (no [Install])", "Install" not in service.sections())
+check("and is not enabled by the package",
+      "enable ekklesia-classroom-reboot-when-idle" not in (root / "packaging/debian/postinst").read_text())
+build = (root / "tools/build-release.sh").read_text()
+check("both are packaged", "scripts/classroom-reboot-when-idle" in build
+      and "systemd/updater/ekklesia-classroom-reboot-when-idle.service" in build)
+
+print()
+print("3. is anything playing?")
+
+
+def cards(states):
+    base = work / f"asound-{len(list(work.glob('asound-*')))}"
+    for n, state in enumerate(states):
+        status = base / f"card{n}" / "pcm0p" / "sub0" / "status"
+        status.parent.mkdir(parents=True)
+        status.write_text(state + "\n")
+    return base
+
+
+check("nothing: no cards at all", not idle.audio_playing(work / "none"))
+check("nothing: a card that is closed", not idle.audio_playing(cards(["closed"])))
+check("nothing: a card that is open but not running", not idle.audio_playing(cards(["state: SETUP"])))
+check("playing: a card that is running", idle.audio_playing(cards(["state: RUNNING\nowner_pid   : 1"])))
+check("playing: one of two cards running (the television's)", idle.audio_playing(cards(["closed", "state: RUNNING"])))
+capture = work / "asound-capture" / "card0" / "pcm0c" / "sub0" / "status"
+capture.parent.mkdir(parents=True)
+capture.write_text("state: RUNNING\n")
+check("a microphone recording is not playing", not idle.audio_playing(work / "asound-capture"))
+
+print()
+print("4. the decision")
+check("half an hour untouched, nothing playing: reboot", idle.should_reboot(2000, 100, 1800, False))
+check("29 minutes: not yet", not idle.should_reboot(1840, 100, 1800, False))
+check("an hour untouched but a video playing: not while it plays", not idle.should_reboot(4000, 100, 1800, True))
+check("half an hour by default", idle.IDLE_SECONDS == 30 * 60)
+
+print()
+print("5. run for real: a keyboard that is used, then left")
+keyboard = work / "input" / "event0"
+keyboard.parent.mkdir()
+os.mkfifo(keyboard)
+os.mkfifo(work / "input" / "event1")   # a second device, never used
+marker = work / "rebooted"
+asound = cards(["closed"])
+holder = os.open(keyboard, os.O_RDWR | os.O_NONBLOCK)   # keeps the pipe open, as a device is
+holder1 = os.open(work / "input" / "event1", os.O_RDWR | os.O_NONBLOCK)
+result = {}
+
+
+def run():
+    result["code"] = idle.main(["--idle-seconds", "2", "--check-seconds", "0.2",
+                                "--input-glob", str(work / "input" / "event*"),
+                                "--asound-root", str(asound),
+                                "--reboot-command", "touch", str(marker)])
+
+
+thread = threading.Thread(target=run, daemon=True)
+started = time.monotonic()
+thread.start()
+for _ in range(10):                       # typing every 0.4 s for 4 s
+    os.write(holder, b"k" * 24)
+    time.sleep(0.4)
+check("while the keyboard is used, no reboot (4 s against a 2 s limit)", not marker.exists())
+thread.join(8)
+waited = time.monotonic() - started
+check(f"left alone, it reboots once the limit has passed ({waited:.1f} s)",
+      marker.exists() and 5.5 <= waited <= 8, result)
+
+marker.unlink(missing_ok=True)
+playing = cards(["state: RUNNING"])
+result.clear()
+
+
+def run_playing():
+    result["code"] = idle.main(["--idle-seconds", "1", "--check-seconds", "0.2",
+                                "--input-glob", str(work / "input" / "event*"),
+                                "--asound-root", str(playing),
+                                "--reboot-command", "touch", str(marker)])
+
+
+thread = threading.Thread(target=run_playing, daemon=True)
+thread.start()
+time.sleep(3)
+check("untouched but playing for 3 s against a 1 s limit: no reboot", not marker.exists())
+((playing / "card0" / "pcm0p" / "sub0" / "status")).write_text("closed\n")
+stopped = time.monotonic()
+thread.join(6)
+after = time.monotonic() - stopped
+# Sound is seen at each check, so the limit runs from the last check that
+# heard it: at most one check (0.2 s here, a minute in a classroom) early.
+check(f"the sound stops: it waits the limit again, less at most one check, before rebooting ({after:.1f} s)",
+      marker.exists() and 0.75 <= after <= 3, result)
+os.close(holder)
+os.close(holder1)
+
+print()
+print(f"{passed} passed, {failed} failed")
+sys.exit(1 if failed else 0)
+PY
diff --git a/tests/test-release-versions.sh b/tests/test-release-versions.sh
index 3fe9409..c751c29 100755
--- a/tests/test-release-versions.sh
+++ b/tests/test-release-versions.sh
@@ -441,8 +441,11 @@ publish("canary", "0.2.7")
 status, out = run()
 check("a device on 0.2.6.1 recognises 0.2.7 as newer and installs it",
       status == 0 and installed() == "0.2.7", out)
-check("and reboots, because the manifest asks it to",
-      (work / "systemctl.log").read_text().strip() == "reboot")
+# Since 0.2.6.5 not there and then: the reboot is handed to a service that
+# waits until nobody is using the computer (classroom-reboot-when-idle).
+check("and asks for the reboot the manifest wants, once nobody is using the computer",
+      (work / "systemctl.log").read_text().strip()
+      == "start --no-block ekklesia-classroom-reboot-when-idle.service")
 publish("canary", "0.2.6.1")
 
 device("canary", "0.2.7")
diff --git a/tools/build-release.sh b/tools/build-release.sh
index 418402f..90d32f0 100755
--- a/tools/build-release.sh
+++ b/tools/build-release.sh
@@ -193,6 +193,10 @@ install -Dm755 \
     "$ROOT/scripts/ekklesia-classroom-update" \
     "$STAGE/usr/local/bin/ekklesia-classroom-update"
 
+install -Dm755 \
+    "$ROOT/scripts/classroom-reboot-when-idle" \
+    "$STAGE/usr/local/bin/classroom-reboot-when-idle"
+
 # System configuration.
 install -Dm644 \
     "$ROOT/config/sddm/ekklesia-classroom.conf" \
@@ -206,6 +210,11 @@ install -Dm644 \
     "$ROOT/systemd/updater/ekklesia-classroom-update.timer" \
     "$STAGE/etc/systemd/system/ekklesia-classroom-update.timer"
 
+# After an update: the reboot, once nobody is using the computer.
+install -Dm644 \
+    "$ROOT/systemd/updater/ekklesia-classroom-reboot-when-idle.service" \
+    "$STAGE/etc/systemd/system/ekklesia-classroom-reboot-when-idle.service"
+
 install -Dm644 \
     "$ROOT/systemd/provision/ekklesia-classroom-provision.service" \
     "$STAGE/etc/systemd/system/ekklesia-classroom-provision.service"
diff --git a/tools/validate-trixie-release.sh b/tools/validate-trixie-release.sh
index 20d405f..b1e1d7c 100755
--- a/tools/validate-trixie-release.sh
+++ b/tools/validate-trixie-release.sh
@@ -145,6 +145,20 @@ systemctl is-enabled NetworkManager.service >/dev/null
 systemctl is-enabled cups.service >/dev/null
 ! systemctl is-enabled NetworkManager-wait-online.service >/dev/null 2>&1
 
+# The update check: daily at 03:00, caught up after a night switched off,
+# and still so after this install or upgrade. The reboot after an update
+# waits until nobody is using the computer, in a unit nothing enables.
+systemctl is-enabled ekklesia-classroom-update.timer >/dev/null
+timer=/etc/systemd/system/ekklesia-classroom-update.timer
+grep -qx 'OnCalendar=\*-\*-\* 03:00:00' "$timer"
+grep -qx 'Persistent=true' "$timer"
+! grep -qE '^(OnBootSec|OnStartupSec|OnUnitActiveSec|OnUnitInactiveSec|OnActiveSec)=' "$timer"
+! ls /etc/systemd/system/ekklesia-classroom-update.timer.d/*.conf >/dev/null 2>&1
+test -x /usr/local/bin/classroom-reboot-when-idle
+[[ "$(systemctl is-enabled ekklesia-classroom-reboot-when-idle.service 2>/dev/null || true)" == "static" ]]
+! grep -q '"systemctl", "reboot"' /usr/local/bin/ekklesia-classroom-update
+grep -q 'ekklesia-classroom-reboot-when-idle.service' /usr/local/bin/ekklesia-classroom-update
+
 # The audio policy reaches a session, and is ordered after the server it
 # actually talks to. Ordering it only after pipewire and wireplumber is
 # what let it race PipeWire's PulseAudio server at boot, fail its one