And so, a short-hair rule that applied to boys and not to girls was deemed a violation of Equal Protection.
The court is not saying that dress-and-grooming rules need to be identical for male and female students. The violation seems to emerge at some point when the rule aimed at boys is too severe and when there's no severity toward the girls. Actually, the court lacked information about the leniency toward the girls. From the PDF of the case:
Although girls can evidently wear their hair as long as they wish, could a female basketball player wear her hair in an extremely short “buzz-cut,” which might literally qualify as “clean cut” but perhaps not in the sense that Coach Meyer means it and perhaps not in synch with local norms? Surely girls with longer hair must do something to keep their hair out of their eyes while playing basketball.... But, at the risk of stating the obvious, boys with longer hair could do the same. In fact, male athletes use head and hair bands to do this very thing, as anyone who has watched professional basketball or football games recently can confirm.As the separate opinion by Judge Manion stresses, the parties litigated the case as if it was all about substantive due process (right of privacy). Manion agrees that the student deserves to win, but only because the school failed to meet its burden to justify the different policy toward males and females. Maybe the school could have established that the different policies were grounded in "social norms or community standards." It didn't try. The Equal Protection aspect of the problem was ignored.
Why not remand and develop the record? The answer is easy and procedural: The parties submitted the case for final judgment with stipulated facts.