You say internet access is though a DSL line. How, exactly, is is distributed? Do they have one of those Linksys/NetGear router deals? Unplugging the DSL line comes to mind.

The problem I see with any solution to this problem is not turning the service on, but turning it off. It's like you need a reverse deadman's switch, but I can't think of an appropriate way to set anything like that up. I mean, what happens when someone comes into the room and tells the chaperone that they need him right now. Is he going to stop and turn the internet off first? If not, you need either some automated facility to detect that he's not there, or it's just going to be left on.