Right I've liked this boy for a long time and we use to be best friends until one day he completely changed into this horrible troublemaker! I thought he liked me back until I heard from his friends that he likes my best friend! What do I do? I just want to forget him but its kind of hard since he is in most of my classes in school :( by the way I'm 15 :)

I know it must be hard, but this a part of growing up. Navigating life when you are in an uncomfortable situation.
You know the guy likes someone else, you say he is a horrible troublemaker, so you remind yourself of this and not let your mind stay fixated on him.
You are better than this. It's not good for you, so find some other fun and healthy outlets

You don't ever forget, but you do other stuff with other people that you like and enjoy. You don't even have to speak to him. In time, seeing him won't even be as big a deal as you thought it was.