This isn't about What is "proper" for school, I think it's about the future of these kids. What these kids fail to understand is that first impressions go a long way. What they think is an expression of their individuality (piercings, tattoos, wierd hair styles, ect) may keep them from getting the job they may want. I know that if I have a choice between hiring a kid who projects a conservative image, and a kid that looks like he went bobbing for apples in a tackle box, I'm going to hire the normal looking kid first.
There are different behaviors for different places. Kids need to learn this to get along in the world.