My boyfriend always mentions this girl that he goes to school with and she is really pretty... he always says 'i like you, she is just a friend' I don't want it to bother me but it does, I don't want him to leave me for her and I'm scared that they will get together! How do I win him?

You don't "win" people. You have more pride than that! How can he "win" you is what you should be asking. Just don't worry about this other girl. You are too young to be thinking about "winning" someone. Don't play games unless you want to get played!

If she is his friend, is it possible to be yours? Have you met? Maybe you should.

If he is interested in her because she is pretty then you shouldn't worry about him at all. If likes you then you don't need to worry about it. Be confident and if this girl is challenging show your metal don't be intimidated by a "pretty" girl! Good luck