Nano, I don't know your school district or their policies, but at my high school anyway, if anyone impersonated an admin or school official (electronically or otherwise) OR if anyone did anything to alter computers, it was grounds for automatic suspension and possible expulsion. We didn't have to log into computers either, but yet kids still got busted for these kinds of things.
My advice: playing VMK a few extra times a week is not worth risking getting kicked out of school over. Just something to think about.