Deepfreeze is ideal for classrooms, and the educational rate is reasonable.
Rapid restore is another great product. Boot to F11 and restore your master image, all changes wiped out. IBM has a great deployment manager that lets you image a system from the server - very very slick.
But you may not have the funding for something that costs money.
If you are already licensed for ghost, you could create a ghostcast server with boot disks/boot CD's. If you set it up properly just booting off the floppy will do the end-to-end image of a students PC once they are done messing around with it.
If you are talking about daily, perhaps locking the students down with policy is your best bet for prevention.