Well, at least the girl can't say she did not get her revenge for not being allowed to go to a prom. On the other hand the schooled also needs to be sued for blackmail. If you guys think back to high school, do you remember how it was and what would have happened if the whole prom was cencelled because of one person? I would definitely call that some form of blackmail.
P.S. Prom is a very not wise idea.