Two things:

1: If you look at the hardware docs on the N900, the back camera is actually entirely contained on a socket-ed chip, and can be extracted with the proper care and tools. That would clearly disable the camera, and remove the chance of using it until it's physically re-installed. The front cam really only works for skype and programs that specifically call out to use it, so that one is less of an issue.

2: Another option may be to just buy a solid rubber back for a device with similar dimensions. Cover the camera, and it won't be seen and can't be used.

Really though, this sounds like a bogus thing. "We don't want our students to use the most current technology." Really? What school are you going to? I can understand if they ban camera phones in some areas, like a gym, or a theater, but a wholesale ban sounds like a lawsuit waiting to happen. (Well, it would in the US anyway. )