I think the watch trying to connect to cellular is the main battery drain problem too. Especially at our current school, cell reception is really bad, so the watch is regularly straining itself to connect to cellular. There's also no Wi-Fi, so that's not an option either.
Friends of ours have had some success with turning Low Power Mode on at school.
The problem with that for us is that it's really difficult for our kid (or most kids, probably) to remember to turn on and off Low Power Mode. If they don't turn it off after school, it can be up to an hour until they can get a text message from us or for us to see where they are on the map. That would make coordinating with both regular and unusual circumstances potentially very stressful.
So what would be most helpful is if Apple added a Low Power option to the Schooltime mode or any mode that we could schedule during the day. I've always thought it was unhelpful that you can only define one downtime per day. With the watch draining so much power all the time, I'd think Apple would want to create as many opportunities as possible for kids with watches to be able to have scheduled impeding times also extend their battery life.