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.
How do girls show they like a guy? Particularly a shy girl
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.