A girl's dress code is clearly unfair compared to a boy's dress code. If you think about it, it makes sense. At some schools, males can show shoulders, but females cannot. Shoulders are pretty much the same on anyone's body; it's just bone with skin over it, nothing special. So why do girls get in trouble for wearing shirts that show the shoulders?
If it was just about the shoulder blades then the distinction is both silly and unfair. If that is the issue I will concede to the pro side. But that is not the issue not as far as I can see. It goes further than just the shoulder blades. I do not see anything wrong for a woman to be punished for wearing a shirt which draws undue attention to the her breasts. Whereas a male student if he had worn a shirt which allowed the same amount of skin to be exposed would not require correction.