Not that it would be a good idea otherwise, but this doesn't work: any enterprising student can simply hash all possible student IDs and find who's gotten what

Not that it would be a good idea otherwise, but this doesn't work: any enterprising student can simply hash all possible student IDs and find who's gotten what grade. (SHA256 is fast.) Yes yes, use bcrypt etc., but that's overkill; I'll post a simpler solution.