I'm going to throw you some random advice I learned from being a shy, awkward high school kid before becoming comfortable with myself and happy (read that how you may) in college and beyond.
1. Be comfortable with who you are and what you enjoy. Be passionate about these things. Most guys who girls run across are virtually the same. Let her realize (without bragging) there is more to you than the others, no matter what the 'more' is. The best way is to make fun of yourself while doing it. For example, I loved reading books about Economists and their theories in college; nothing is nerdier than this to a girl. If the conversation permitted, I would bring this up and tell them what a complete nerd I was for loving this kind of stuff. I'd make a joke about getting together and reading some theories together sometime. After this, ask her if there is anything nerdy about herself? If she starts talking, shut the hell up and listen.
2. Shut the hell up and listen. Keep her talking. Ask her questions to keep her talking. Make small jokes. Nod. Ask more questions. The more she talks the more she will like you.
3. Make fun of yourself. Be sarcastic. But don't be a dick. Don't be arrogant. And don't make fun of those less fortunate than you.
4. Have fun with life. Even if she turns you down, tell her you still think she's amazing and you're just happy with yourself for getting the nerve to talk with her. Ask her if she'd be willing to turn you down on a weekly basis so you remember crashing and burning is not really that bad of a thing. She'll think you're funny and, who knows, maybe one day she won't say no.
5. Final advice. If you're looking at her a couple desks away, there may be some amazing girl several desks away looking at you. When I made this realization for myself, I met and dated some of the most amazing, deep, and beautiful girls a man could come across. I know you're only 14, but, trust me, there is nothing sexier or more beautiful than a girl who is comfortable with who she is and what she wants. I'm generalizing, but sometimes the "pretty" girl is not always the most comfortable in her skin.
Take this or leave this, I don't care. But this worked for me and my life was better because of it.