Bully

Simply complete 9 classes to unlock this achievement. Classes run from 9am-12pm and 1pm-3pm and are shown as a bell on your mini-map. If you're lazy like me, getting busted during these times will result in being dropped off at the current class by the prefect.

This page has some good guides to the lessons if you're stuck with any:
http://www.giantbomb.com/guides/achievements-classes-and-err...