The dress code refrains you from the first amendment which even in schools you should have the rights. I mean it's not like we get shipped out of the USA every time we step onto school grounds. And its sexist. We arejust forced to do things that we might not want to do.
At a public school (trust me I know about this) kids get bullied for their clothing all the time. If you wear un ugly sweater kids will be mean. Its just what they do. Uniforms allow kids to be equal therefore taking away that disbalance in power caused by different clothing.