I don't want to take this one girl to the prom after all.
Why don't you want to take her to prom?
I'm kind of interested in another girl
If you haven't already agreed to take her to prom, you are under no obligation to do so. You should let her know you already have plans, and ask the girl you are interested in if she would like to go to prom.
I'm just kidding. I do want to take the first girl. I wanted to just see how you would react is all.
Oh haha, I didn't realize it was a joke. I should be more aware. Evenstill, it sounded real to me so hoping you are truly joking.