I agree with Charles. Don't try to be too witty or have the perfect line ready; that is just cheesy.
I understand it can be tough if she is with her girlfriends all the time, but you simply have to show up. A good idea might be to ask her friends if you can talk to her friend in private for just a minute. If any of them ask why (they might, considering you don't know her yet) you can be bold and cut straight to the chase. "I think you are cute and I wanted to talk to you alone for a second"
Does she have Facebook? Add her and chat with her there! Chat with her a bit casual, and then when you approach her in school you will not come on to her as strong.
I am just spit balling here! Let us know what you do and how it works out
- Andreas