... woah. Well, first things first.
Report him. These things are not allowed in school. Or anywhere for that matter! Then stay away from him. If you're in the same class, possibly ask the teacher to keep an eye out for him or even ask for him to be transferred or something. If you feel uncomfortable then it's best to tell people and make sure they know.
Another thing! It doesn't matter if he has a girlfriend, or not, this is entirely wrong! Who cares if he's "cheating", you shouldn't let a guy do anything to you anyway. Keep an eye out, don't do anything you don't feel 100% ok with.
Good luck! And sorry if my comment went haywire.