Yeah, having a separate table for student-task level is a right way to go here.
Regarding columns, yeah, I guess you’ll just have to do any schema changes by hand. I don’t think you’d have much benefit in adding columns automatically though.
Think of it this way. You’re the doc maker, and the lower tiers are doc users. Your responsibility is to design the doc in such way that 1) for users it’s obvious how to use it, and 2) they never have to (and never can) touch the setup and the schema. So ideally your teachers never have to e.g. set up the filters and sorting themselves (instead you’ll give them external controls for that), and they never can add columns themselves — all these changes should come through you.