If there's a school function coming up soon, like a dance, ask him if he's going with anyone or tell him you wonder if anyone's going to ask you. Work the conversation up to ask if he likes anyone. Regardless of whether he says yes or no you could just casually put in that you like him and just wanted to let him know.
The only way you'll find out is if you ask him. And he'll have way more respect for you if you ask in person rather than a note or text or having a friend tell him.
Edit: Just wanted to add that adam_89 is right. A lot of movies and TV make a point of it being this huge scene if you tell someone you have feelings for them, but it doesn't have to be that way. Just keep it casual and it won't be awkward
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