Did this happen AT school. If so it sounds like a lack of supervision. Pass the responsibility back to them.
If not, there would seem to be little you can do. It is not an offence to change your name UNLESS it is with the intent to deceive which could apply in this case. If the people involved are over 18, which I doubt, then the police might want to become interested, if not, then a friendly word to the other person from a man in a blue serge suit and a pointed hat might just work.
You could also contact MSN and see what they can offer, after all, its their service that is being abused!