Ok so something I thinks really cool is to go in her house (with her parents permission!!) and put a trail of rose petals from her door to her room and put them around her bed or dresser or something and put a card asking her directly, or maybe a "meet me here at this time" card and when she gets to the designated place get down on one knee with a rose and ask her then
Try to talk about regular stuff like how are you and stuff like that, and at some point throw in there " i really love you, and i just want to eat dinner with you tonight with some dancing in-between, would you like to make my dream come true tonight?"
Kay first of all..... Are you popular? If so try and get the cheerleaders to spell out prom at the next football game.........or go the easy way buy her flowers and get down on your knees and say _______<whatever her name is> will you go to prom with me?
Be up front with her don't be afraid the worse she can do is say no, so go for her and if by chance she says no don't go insane in front of her save that for someplace else like home or when your alone