My wife went to a Christian high school. The girls were required to wear dresses and the boys pants.
The Bible says women shouldn’t dress as men, and men shouldn’t dress as women (Dt. 22:5).
Among Jews and Muslims, it is thought a man should have facial hair.
Long hair is a indication of being female since men often suffer male pattern baldness. Similarly, being bald-headed is masculine.
We have freedom of religion, so I don’t know if our government should be passing laws regarding dress, grooming and manners. But, certainly, private schools can, and should.