Answer:
do it in stages..
start with putting a note in her locker from "a secret admirer". a day or two later some flowers\chocolate. next you send her a note to meet you somewhere nice like a park or the beach. use a friend to help you and set up rose petals that will say "prom?".
you can even make it a date before prom.. it might make prom more fun later :)
too many chickflicks for me :D