Dude,
There's two ways you can go with this.
Either, ask her straight out - the chances of this working are really highly dependant on your relationship with her so far. If you're already kind of close, go with this, if not, then i'd go with option two.
Option two, you open her, DHV (whichever method you prefer to use, but make it good), then make her qualify herself to you. Something like, "So what makes someone like you a good prom date?" Don't explicitly mention you're looking for a date. If she asks, "Why, are you looking for someone to go with?" reply with "No, I'm sorted, just wanted to know what sets you aside from all the other try hard girls, you know, the ones who spend a gazillion dollars on a dress only to realise their frat. boy date isn't really worth it, so shoot."*
Chances are, she'll begin reeling off a few tenuous reasons after having given it some serious thought, and there you have her in a great position. She is standing there waiting to be proverbially swept off her feet, spun round, penetrated and destroyed^ after the prom. Of course, you then turn away as if not impressed, take a step away, turn back and say "Hey, I was actually lying, I haven't got a date yet, fancy tagging along with me?"
This comes across as pretty laid back, and has enough feigned disinterest to make her believe you're not bothered by the outcome, whatever it is. Likelihood is, she'll
want to go with you by this point, gold.
Oh, and also, note how you never actually ask her - you're in control if she complies and goes with you because she's
tagging along with you, and if she doesn't agree, it's her loss, right? You never asked her, you're gonna have a ball either way, in fact, not going with a date leaves you free to explore everyone else's girl right?
* - This kind of acts as a pre-meditated boyfriend destroyer. Think about it, if she's contemplating going with someone else that she likes, chances are he's either a dick, sporty type, frat. boy type, right? Also, if she ends up going with you and you've done this all correctly so she's chasing after you, she's going to reel off to her friends what you said, as her own words and instantly, you've got higher value in all her friends' eyes because you've already distanced yourself from the normal rowdy crowd.
Secondly, you initially disqualify yourself as an option, so she's way more likely to have her defenses down and qualify herself to you because you're already taken. This doesn't mean she's doing it without fear of attracting herself to you, as that comes naturally by qualifying herself, but she's doing it because you're not coming across as needy. Also she's subconciously competing with your date, fighting to become the best woman, when really all she's doing is making herself more and more attracted to you. Win!
^ - Thanks Family Guy.
PM me if you want more detailed help.
Wilde
CHAT