Legally speaking schools basically have leeway to enforce any dress code so long as they apply it evenly and can demonstrate they are not deliberately being selective in application. I guess a case could be made that if they had a dress code stipulating suits for males and dresses/skirts for females they'd have a pretty solid case. I did think it weird they'd be challenging only on grounds of two girls going together since showing up to socials stag or with friends of the same gender is a pretty routine thing, as is girls dancing together. Of course the situation still seems intensely stupid to me and their motives are fairly transparent but it does change the legal situation I should imagine. And I can easily see even fairly open-minded people who might have sided with her getting ticked at legal action and just slamming the book shut with a "Fine, we'll just cancel the whole thing anyway, saves us the expense and usual wad of alcohol and date rape issues"