Please help me! I don't know what to do!!!?

There's this girl at school, and I guess we kinda' flirt with each other, we're sorta' friends I guess, if she's walking in corridor our eyes both light up as they make contact, I really like her. The thing is...I'm scared if I ask her we'd ruin our friendship ya' see and if things don't work out that'd be -1 friend...she's not the most glamourous girl in the school but I think people would be mature enough about it.....