@Nightrunner, if you want to keep your friendship with that girl intact and have a shot at her down the road, back right off now. I know that she seems like the one or whatever, but you're 15... in the grand scheme of things you're only a few years out of puberty. Trust me, when I was that age I was in a similar boat to you, but you would not believe how much you grow and mature as a person every year when you're in your teens. Your body's figuring out it likes girls and it's new and exciting, but seriously do not push the issue. 

The fact she's leading you on like that is not good for either you or her boyfriend, who as far as I'm concerned has done nothing wrong. Have you ever considered he was sizing you up because he thinks you're butting into his relationship and maybe trying to break it up? What would you do if the situation was reversed and it was him trying to get between you and her? You'd probably want to kick his ass, and fighting him is NOT going to win her over or prove anything other than you think with your fists and are quick to anger. The more you push, the more you're going to drive her away, it's that simple, and trying to break up a relationship because you fancy a girl is a dickmove.

Trust me, you have a lot of growing to do between 15 and 20, you're still young and there's going to be countless dating opportunities down the road. You and everyone you know are going to change a lot and it gets to the point where you may not recognize them by the time you graduate. So take it easy, enjoy your time in school, and don't worry so much about finding the "one". The best relationships are the ones you stumble into by chance, not force to happen.

Carry on.