Answer:
Well, either you do or do not want to go with him. You haven't given us that information. If you do want to go with him, just say, "yes, I would like to very much." If you do not want to, then you really need to think about how to turn him down gently. You don't want to hurt him. If you had no plans to go to the dance, then you will have to think of something such as prior commitments (family, friends, etc.) If you are going to the dance (maybe with some friends), that would also fall under prior commitments. If someone has already asked you, then that is easy; "I'm sorry but someone has already asked."