Just walk up to her, slap her ass, and be like yo baby, how bout you and me friday night?
Actually that definatly wouldn't work unless shes the school slut. I'd suggest Lenny if your not already, befriend her. But don't let it get to far as friends before you ask her out, that could be hazardous. It's all about flirting, if you can flirt w/ her, and she flirts back, you know it's a done deal and you can safely ask her out.