The 7th Circuit Court observes that "Coach Meyer’s policy prohibits far more than an Age-of-Aquarius, Tiny-Tim, hair-crawling-past-the-shoulders sort of hair style..."

"... it compels all male basketball players to wear genuinely short hair. In 2014, it is not obvious that any and all hair worn over the ears, collar, or eyebrows would be out of the mainstream among males in the Greensburg community at large, among the student body, or among school athletes. (Even one or two men on this court might find themselves in trouble with Coach Meyer for hair over the ears.) We certainly agree that the pedagogical and caretaking responsibilities of schools give school officials substantial leeway in establishing grooming codes for their students generally and for their interscholastic athletes in particular. But that leeway does not permit them to impose non-equivalent burdens on school athletes based on their sex."

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.