Oh boy, Mac, sounds like you're in for a whole heap of fun! I've done many of these showcase events, and like everything else in theatre, they are as much fun as you make it!
I take it this is a one night event, if that is the case, it does make things a bit easier. I ASM'd a school showcase that did the wristband check in We had the teachers submit the parent volunteers they had from each class. The music teacher (who was in charge of running the event) then provided any other names that needed to be backstage. The afternoon of the event we had a mini tech dress which is when we ASM's went out and wristbanded all the adults who had to be backstage. The theory was, if you missed the tech dress - your backstage duties were handed to someone else. This meant that not only did we get all the backstage logistics figured out, but we also had met and "branded" those who were to be back there, leaving only a few people to be checked in come call time.
Any parent who questioned what/why we were doing what we were doing was gently reminded that this was a security precaution for the safety of the children involved (let's just say this reasoning quieted the overbearing stage mom riding the SM's back!) A week before the showcase, we had the teachers send out info letters to all the kids and parents letting them know about the limited backstage access so the evening of we weren't bombarded by waves of parents wanting to pick their kiddies up. Each class had a designated "parent pick up" spot in the lobby. Each class had made up a sign they put up so parents could easily find them.
It worked for us for that event. I understand that they are still using that system now, a few years later. Ultimately, yes, the principal should be helping you with the whole access control thing, but they are usually busy people so they may just be looking to drop that burden on you. Don't let them, because ultimately the safety and security of those students is their responsibility, not yours. Offer up a few ideas of how you're thinking of doing the access control and get their thoughts on what will work. Definitely once you've decided on what method you're going with, let the students and parents know, this will save you massive headaches the night of! Clearly indicate where the kids should be dropped off, whom to contact if you must get to your child ("family emergency and we have to get home, now!") and how to pick them up at the end of the night.
As for the logistics of running the event, you've got the right idea, I wouldn't panic about that. Use the teachers, use the chaperones, use any crew you might possibly get. Hopefully you are getting a full day tech dress with them. If you do, try to run it in order, getting the classes doing the full move from balcony to backstage to going on so everyone knows how long they need/have to complete this feat. My biggest piece of advice is to try to find some way that those kids don't have to stay up in the balcony the whole time. What you're going to get is a whole bunch of anxious kids who definitely don't want to be stuck up there. Yes, the audience will be comprised mainly of their parents who should understand, but there will inevitably be the parents who get a bee in their bonnet because the kids upstairs were creating a distraction while their precious star was on. If space is extreamly limited, consider having a rotation to the lobby, or rehearsal hall. Some place the kids can run around a bit and burn off some energy/boredom. There will also be a lot of "I gotta pee" moments, so have the chaperones prepared for that (nearest washroom is...)
My biggest tip on a school showcase...barf buckets, both sides of the stage and where the kids will be lining up to go on. Trust me on this one
And as the others have said - if this is too much to be worth it, get out. These events are no picnic, testing you in ways you couldn't imagine. But sometimes it does work out, and either way you did your thing and at some point you will see some kid doing something funny, guaranteed! PM me if you want to talk more about these, like I've said, I've done a whole bunch of these. Best of luck!