I would suggest being creative. Most girls today still are impressed by creativity and chivalric behavior. Therefore, here are a few suggestions for you to try.
1. Write her a letter/poem. (Be careful! Girls can also be vindictive and she may show it to her friends.) You can use classic love poems, or poems about romance, to give you clues as to what to write about.
2. Leave her an anonymous note and flower. Keep her guessing about her secret admirer. She may like the idea of the secret admirer and become very curious about who he may be. After a while, let her in on the secret.
3. Be direct. Some girls simply like to be asked out face-to-face. Here though, you risk being turned down face-to-face.
4. Become friends with her, if you are not already, and really get to know her. As your friendship grows, you will be able to gauge if she is interested.