Eh. Half the people at my Formal were year elevens. Ugh. Ferals that they are. I think if we can deal with having like fifty feral little year elevens running around, you should be good with your boy. >.>
Also, seriously, don't worry about what people say. Just because you take someone to Formal, doesn't mean you like them. Like broadway said, he wouldn't have asked you if he was already "claimed" by someone else. It's just common sense and she's just a jealous, bitter old hag. ;D
I think girls are completely nutso. I wouldn't worry about it - although it doesn't seem like you are. If he had really planned to go with this other girl, he wouldn't have told you that you two should go together. Commen sense people.