How do i tell my crush that his girlfriend is cheating on him?

First you have to get proof because he won't believe you, it'll just be your word against the school tramp's word. If you have proof just tell him and show him what you got. If he knows you like him he'll think it's a way to break him and his girlfriend up. Or you could just let him find out on his on. You might get caught in drama if you tell him.