I think I might be able to help you because it seems that every girl I have ever liked has had a boyfriend. And well it makes sense, because the good ones are always taken, and if they're not, they get picked up really fast.
This is what you need to do. Keep this girl on the back burner. Become good friends with her, and go out looking for other girls. While you're being friends with her, make sure you show her some kind of trust and she'll show you trust back. Like have personal conversations with her, and let her tell you all of her problems. example: shit her boyfriend does to piss her off. I know it sucks, but you gotta stuff your balls inside your chest cavity, and be a good listener. Now you'll be able to learn from these personal conversations. Never say anything to put her boyfriend down though. Here say stuff like "awwww, yeah" and "why would he do that?" -- those lines are golden. Learn ways to make her happy, and soon enough she'll wake up and say "wow, you're such a nicer guy than my boyfriend"... and will be screaming your name like a gambler at the dog track.
Also remember to take other girls out on dates, and then you'll have something to talk about. "I just don't know why stuff never works out with anybody I date". Normally she'll follow that up with an "AAAWWWWW, but you're so sweet."
Trust me bro, highschool girls melt like butter at this shit. As long as you have patience, it should work. Worked for me.