You can restrict iPads running iOS 6 to a single app via Guided Access:
but that requires reconfiguration of the iPads whenever you want to set it up or deactivate or change it. There may be a mobile device management (MDM) system that would allow you to push out such changes "on the fly" but I can't say with certainty. Someone else here or in the iPad in the Enterprise forum may know.
Otherwise it may be necessary to implement an administrative solution such as banning use of iPads in class or enforcing consequences should a student be using the iPad for inappropriate functions.