can we talk about how stupid some of the dress code is?

so like I'm done with school right now, but this just makes me mad when I think about it. so basically alot of the boys in my school own shit that says "virginity rocks" and "I love hot moms" that they wear all the time. they also sag their pants to the point their underwear is showing and no one says anything. there have been boys taking off their shirts before and no one said anything, no one cared. but the one time a girl in my band class wears a shirt that shows a little bit of her back, not even that much and she also had a jacket and was wearing it almost the whole time, my male band teacher dress codes her. my math teacher was confused, even the secretary was. my other friend also got dress coded for having a small knife necklace (it was dull and not even like dangerous at all). 

I just don't get it, like I'm not trying to say there shouldn't be any dress code, but why is it so fucking sexist? I'm so done with it ngl.