Urrrrr proms I'm fed up with ours! Not only does my tux not fit (it's had to go back for adjustments *tuts*) but I got asked out to the prom by a girl and I had to say NO, so me being all soft felt guilty and horrid for days.
Anyway, my advice:-
1. Chocolates - No, no, no, no & NO. Such a cliche. You'd be best asking her if she wants to go out to lunch or just take her for a coffee.
2. Whilst eating/drinking coffee, tell her you like her more than just a friend and that you'd like to go to the prom with her. Make it clear it's not an obligation. Say something like "I'm not sure if you feel the same, BUT...."
Something like that. Good luck!