Girl wants to ask me to Sadies?

this girl wants to ask me to sadies, its a dance at our school where the girl asks the guy, but i dont really know her and we have only hung out like twice and never said a word to each other. And she's not the best looking either and her best friend is asking me if i would want to go with her and i wanna say no but i dont wanna be a . Anyone have a way to say this without being an a$$hole