She does sound very confusing because she is so playful. It seems like you can't take her seriously because first she tells you she doesn't have a date, then she says she is going with Joey. Right now you can't really tell if she is interested in you or not. I'd say, find another date to take to the Winter Formal. If you do dance with her for one song, maybe you can gage whether she is interested or not, on the basis that she is more serious at that time.