Your brain is really only very good at focusing on one thing at a time. If you occupy it with something else while this person's around, it'll be too busy to pay much attention. This works well in reverse too - if you really have to pee, think about sex to put your bladder on the back burner.
But to solve your current quandary: If you have deep, scarring emotional problems, try wallowing in them while this person is around or is in your thoughts. If you don't have such problems, get a hobby or a cat. Preferably two.
Crushes happen because we're wired for them to happen. If you really think this person is the super-best-ever thing that could happen to you, then you need to go over and introduce yourself. If not, then just think about baseball or schizophrenia until the end of class ;)
Hope this helps!