Well, the administration and school board must be quite strict on the school dress code. You could classify it as maybe being sexist, but I believe they just want to keep it appropriate. But then again, I also believe that it's sexist, because how come the guys can wear whatever they want?
It's balanced here, in my school. It's private, and both the guys and girls have to wear uniform. (It's an islamic school.) And if I were to be honest, the girls have slightly more freedom. The guys have to go for a light blue polo shirt with the school crest, and navy blue pants. No jeans, no nada. Girls are either long jilbabs and a white scarf, but there's also a uniform with the school crest. While they guys only have a freedom of shoes, and only during P.E., girls get the chance to wear multi-colored hijabs. But that's not the point.
Girls have certain body parts, and the school may be strict in having them covered up, though boys do as well, and they should be covered appropriately as well. But since girls have certain body parts, that makes it so that the school is quite strict on them, in making sure they cover everything.