Dress codes for girls are extremely sexist. My schools that I've been to allow boys to sag all they want but if a girl shows the smallest bit of strap(even if it's by accident) it's the biggest deal in the world. And I know that there are a bunch of other places that are exactly like that
My school has a dress code that applies to both genders. I have personally seen both boys and girls get called out by the teacher for violating it. There should be dress codes that prohibit inappropriate clothing.