Use a good lock.

Lock up properly (frame and both wheels to a very solid impossible to cut object).

Move bike around, as somebody suggested.

You should be fine. Where are your classes? Is it at a college/uni or some military academy? If it's on the army premises, I wouldn't worry too much about it, I guess (although recently a local paper featured a story of a bicycle being stolen from the parking lot of the police headquarters... but that's Toronto, a runner-up for bike theft capital of North America )