There's a rumor that my boyfriend cheated on me with a girl at school?

Unless you can find a way of finding out for sure, I would leave him. Just to be safe, you wouldn't want to be the only one who doesn't know the truth if he did. However, you know him best and as he is your boyfriend, you should be able to talk to him about it. If she is lying then don't let her ruin your relationship, because the only reason a girl would do this is probably to cause a break up.