Garmin Bounce[0] for kids, which is the watch which directly competes with this, has a 12-24hr battery life on LTE, although Garmin claims 2day battery.
[0]: https://www.dcrainmaker.com/2023/01/garmin-bounce-activity-t...
[0]: https://www.dcrainmaker.com/2023/01/garmin-bounce-activity-t...
Seems like the energy used to keep the LTE connection up is the main problem, if all the users have the similar settings to minimize battery used by the Garmin Bounce.
If Google/Fitbit says the ACE LTE only lasts 16 hours, I'd hope their estimate is much more conservative than Garmin's and has a lot less variance. Otherwise, I can't see the ACE LTE lasting until end of school day.