What do I do if the guy I like has no idea I exist?

Imagine, in a few years your greatest regret would be not talking to him. You'll spend countless years wondering...what if..? What would have happened if I talked to him? You have nothing to lose. March up to him the first chance you get. Their are a million things to talk about. For instance: Ask him if he studied, if he thought the test was hard, what he's doing this summer, and there are so much more! If he is a guy that's worth it it will be easy to keep the conversation going and you wont have to worry about running out of things to talk about . Worst case scenario: He doesn't seem interested,well oh well at least you tried and you went spend the rest of your life wondering. Trust me I've been there, in fact I AM there and I feel nothing but regret! You are too young to regret! DO IT! And good luck :)