Well I don't think I'll be able to give any helpful advice because so far in life, I've had NO luck with guys, and I just told the guy I liked that I like him and of course he rejected me! Hmm maybe that's why I have zero self-esteem and confidence!
Like Ashley said - Even though you may get rejected, it really does feel a LOT better once you've gotten it off your chest! And look at it this way - the worst thing he'll say is no.. You'll live.

But anyways, I agree with guster girl. Maybe you can casually start talking to him? Like, "How's your day been?" or "What do we have for homework again?" (if you're in the same class). Just try to start by asking completely harmless questions... then maybe ask if there's a way to talk online, like email or instant messenger... then you can get to know him more, and if you're comfortable, eventually tell him how you feel. Good luck!