Dress codes in school make perfect sense; there are dress codes in the workplace and in stores. However, recently dress coding students has become a way to stop (usually female) students from showing any amount of skin, or wearing clothing that is form fitting in any way.
A line needs to be established between professional, and ridiculous. No one should be told to change for wearing leggings under a skirt or for wearing a strapless dress even though there is a cardigan over it.
This kind of treatment oversexualizes students, and although it is a stretch, it does also incubate the first ideas of rape culture, that a woman's body is shameful, and not to be put on display.