Pulling random numbers out of the air time...

Lets say 75% of the students are online at any one time. That gives us 108 students. Minimal service I'd say would be 56k dialup for downlink anyway. That gives you roughly 6Mbps just to get those 108 an equivalent speed of a 56k modem. You'd also need to traffic shape the crap out of that link to stop a handful of people hogging the entire link.

How much is bonded DSL in that area? Whilst you may be too far for 6Mbps over a single line, can you get >6 Mbps over multiple lines? Install a proxy that blocks P2P or throttles it down.