Ah, prom....
If I could go back a couple of decades and ask Mary Zetterberg to the prom (my biggest regret in high school was not asking her), I wouldn't do it in any of the myriad ways I imagined back then.
Dress decently, be clean, put your best foot forward without going outside your "image", and talk to her. Tell her that you'd be really honored to go to the prom with her, and ask her if she'd like to go with you. Simple. Direct.
Of course, be confident, smile, be personable - but no need to do anything "exotic".
What I've found is that the way you ask a girl out - if she already knows you, as presumably this one does - doesn't really matter, as long as you don't make an idiot of yourself. She either likes you enough to go with you or she doesn't. If she does, she'll appreciate it if you're confident and don't hide behind a gimmick - and she'll say yes. She would have said yes to a gimmicky question, too, as long as it wasn't too extraordinarily goofy, but she'd prefer that you ask normally.
She already knows if she wants to go with you. Really. Just ask and find out.
