I don't know that we've heard enough about the specifics of the case.
I don't know if the legal fuss being made is predominantly about the freedom of association issue (who she was going with) or the freedom of expression issue (what she was wearing). If it's what she was wearing, I think thats getting towards the frivolous side... however, I will say that its one thing if the school stops EVERYONE from wearing something, its something quite different when they stop one person doing something while others are permitted to. If the boys are allowed to wear tuxs, I can see some merit to the idea that not allowing a girl to wear one is a bit of a double standard. I appreciate its not traditional for girls to wear a tuxedo, but you can hardly say its "against the dress code" if approximately half the people attending the event will be wearing the same thing.
Let me see if I understand this correctly. Since half of all those in attendance will be wearing a tuxedo, and this half is exclusively comprised of males, and this is dictated by the dress code all males in attendance wear a tuxedo, but no females, then a woman wearing a tuxedo is not against the dress code? This makes absolutely no sense at all. This conclusion does not follow from the premises at all.
Rather, the female wearing a tuxedo is against the dress code precisely because A.) all males in attendance will be wearing one B.) all males in attendance must wear one or can wear one, or wear something very similar to one and C.) no female attending may wear a tuxedo, where A, B, and C are dictated by the dress code, or some combination thereof.
Furthermore, this commentary of a double standard is equally as intriguing as the allegation of stating there is no violation of the dress code. The reasoning to support the notion there is a double standard is the fact males are permitted, and only males are permitted, to wear tuxedos. Well, where exactly is the double standard? I suppose we could, by following your logic, conclude there is a double standard if the men were precluded from wearing dresses to the prom but not women. This entire notion of a double standard is resting upon the principle of if the sexes are not treated the same, then we have a double standard. Yet, I reject this principle because a double standard does not exist on the mere basis the sexes are not treated the same, and in this instance the act of not treating the sexes the same is the dress code prohibiting girls from wearing a tuxedo to the prom whereas boys may. However, this does not lead to the conclusion there is a double standard.
Now, regarding the legal complaint. The complaint, prepared and filed by the ACLU, alleges a freedom of expression violation. In order for this claim to have any viability, there must be speech or conduct akin to speech. The ACLU alleges in its complaint the conduct constituting as speech in this case is A.) Constance will be expressing she is a lesbian by bringing her same sex girlfriend to the prom and her social or political viewpoint it is okay for gay and lesbians to bring same sex dates to the prom. B.) Wearing a tuxedo expresses her view point it is okay for female students to wear tuxedos to the prom.
I have not really had much time to ponder either one, much less engage in any legal research on these issues yet. However, the above is the basis of her complaint.