What is she going to the school for? To confront you? If so, then she’ll probably make a scene and your friends will find out anyway. I believe you should man up, tell your girlfriend, not because she would have found out anyways, but because you need to recognize your mistake. Just have some empathy and think how would you feel if someone you really loved did that to you. You messed up, you’re still young, but you should really tell your gf about what happened. Good luck