yea, 4ft reef would do. Maybe a 55. Can't imagine much more room in classroom.
Get a bunch easy corals and damsels just in case one of the students drops a hit of ecstasy in there
Smaller financial hit.
I think that's big enough to add some interesting creatures. You wouldn't have to go too crazy with cost.
just be careful of the janitors when they clean the glass. They have spray bottles that might get cleaner in there.