Shy girls get timid at first then make a point to single out the guy in conversation. Maybe pick on him a little and say stuff to make him smile. If the guys paying attention it's easy to pick up on.
Basically all you have to do is say so. Teenage guys are shy so they don't get that a lot. If you like the guy...say so...because I was a shy guy and I literally could not catch a hint. I once had a girl grab my arm in a sexual manner and didn't realize it until a day later. So if you really like the guy say so to his face because if he's shy then he's too stupid too notice your affection
Give him a innocent shy look. I'm shy around guys.