Building actual software for actual use cases for actual people is extremely hard, and a lot harder than it feels like when you're just a few visionaries with an itch talking about these things. I'll say it again, slowly. It is EXTREMELY hard.
Sure, you can probably get a couple of enthusiasts together and build an infinitely better 80% solution in Ruby on Rails over a weekend, but you will learn the hard way just how important those last 20% are. That unpleasant senior teacher that never liked you? He uses a feature in those 20% and he's friendly with the principal and the union rep, and before you know it, you're up to 3am, not building awesome solutions for the entire school, but building bespoke solutions to that guy's convoluted backwards workflows that barely made sense in the 70s when he came up with them -- and you don't have the political capital to tell him to shut up and get with the program, however warranted that message would be.
If you do pull through, your reward will be a pat on the back, and a lifetime of being on 24h call.
Oh, and that's your best case scenario. There's a high risk of not actually achieving a system that is substantially better than the legacy system, even in the 80% case. And a substantial risk of introducing a subtle bug that screws up grading at the worst possible time (you know, how an administrator pulls a transcript giving an A+ student a B- average and sends it to his prospective college, screwing up his future, and the error is only caught after the deadline, and it's all literally, personally your fault).
To pull this off, you need a team, a budget AND hard executive sponsorship. That sounds unlikely for a public high school.
If you think you have the vision and the domain knowledge, and the technical chops to pull this off (but no budget and/or no executive cover), your best bet might be something like YC: fix this problem for ALL schools.