Okay, since you know that the she will for sure say yes your problem is now that you want to bust the question in a classy way. Someway that will make the prom memorable...

The Monk says: "Worry not about how you are going to ask her. The simplest way is always the best way. See her before school, or preferably before her first class of the day. Walk up to her with a single red rose (from the store or a garden). MAKE SURE that the rose is NOT wrapped up all crasy like with babys breath or that plastic crap. Just one single long stem red rose.
Tell her something something like... "Ive been thinking a lot about this prom. I want it to be a special one. There is only one special girl who I want to go with me. (look into the eyes, extend your hand with the flower like you are giving her the flower) That girl is you."

YOU ARE SO MONEY AND YOU DONT EVEN KNOW IT!!!
Lemme know if The Monk can help you with anything else.

 

The monk knows what he's talking about .
I suggest you do that, guaranteed to work everytime