Have you talked to your school's tech department and network admins yet? That should be the first step. They can trace things a lot more effectively than you might be able to. Plus they also have the administration on their side, they can tell the student to do what needs to be done or suffer, you on the other hand can not deliver such justice.
Edit: Also, how are you sure it is another student and not a router or switch that has some settings that have gone screwy? It could be 1 of 1000 things.