How do you tell a guy you go to school with that you like him and what if he doesn't like you?

From your question I would assume you've not spent much time around the guy, since yo're not sure if he likes you or not. But I could be wrong. First of all get to know him before you even tell him you like him, say hi to him and strike a conversation for instance a subject in school that you like or he likes. The more you conversate with him the more you will get to know him. It is not about knowing how to tell a guy you like him is about getting to know him first and only then he will answer your question without you asking it:-)