I never disputed the fact she had standing, so your reference to it is rather puzzling.
Second, this analysis is much better than the prior lame allegation they were violating freedom of association and expression. Rather, your remark above is asserting the planned school event was cancelled based on the content of her expression and freedom of association. This is a much better claim and argument to be made.
Typically, the government cannot censor a message based on its content, unless it falls within the well defined and long recognized exceptions of A.) slander B.) libel C.) incitement, Brandenberg v. Ohio, D.) threats or E.) regulation/censorship, not because of the content, but the effect of the message (Tinker v. Des Moines).
Here, we have to assume censorship exists by the cancellation of prom. To be sure, the young lady cannot express herself, or her freedom of association at the prom, but I question whether cancellation of it constitutes as censorship of her message.
In addition, we have to further assume dating someone, or taking someone to the prom as a date, constitutes as freedom of association and I am not sure this is true. The caselaw in this area has never involved just two people but a considerable group, such as clubs, organizations, fraternities, etcetera.