Actually, students who move frequently are encouraged to keep their instate home address registration and request absentee ballots be sent to their away from home school address. If they are living in a different county than their parents, the parent may even request that the absentee ballot be sent to the student at school on their behalf.
If a student registers at their school address in spring of 2012 and then moves next fall, they will need to update their voter registration in order to vote in the Presidential election - not necessary with absentee ballots!! That said, the most important thing is to register to vote (wherever you consider home!)