@markd Thanks for the response!
Making a phone app seems like it would work, but it’s also incredibly complex. In this case the phone is simply acting as a storage mechanism which we can not access. I’m wondering if there are other SaaS services which will do the same thing: provide a storage which we (developers) can not access, but each user can. Unfortunately I think it would have to be connected to authentication, because otherwise the app would just send a user ID to get the private key, and this can be faked by us easily.
The other option I’m considering is simply giving the Auth0 account to the school IT department (grossly incompetent), and hoping they don’t decide to play around with the dashboard and screw things up. I might even give it to a teacher I trust instead if that satisfies the administration, as teachers are much more trustworthy than the IT dept 
. According to the school administration, once I graduate (in a few years) there will be no restrictions on being able to view grades of other students, so I can take over hosting it again. I’d rather not do this unless absolutely necessary though.
Thanks for the help anyway!