What is your point?
Teenagers are only partly-formed humans and can't be expected to have the perception, discretion, or understanding of adults. They want more than anything else to be popular, and are willing to do whatever is necessary to be popular. That includes wearing the appropriate clothes, adopting the current "hip" jargon, mocking non-conformers, and associating only with people considered "cool," (if they will have you).
Like it or not, "stereotyping" is a valid form of learning by experience (just like "profiling" in the context of criminal justice). If you see people with a certain characteristic and they all (or mostly all) behave or think in a certain way, then you logically conclude that they are "all" that way. There is not enough time in life to interview everyone who is different from yourself to see if their views are exactly what you have guessed them to be, based on race, religion, association, neighborhood, or whatever.
When I was in high school, the biggest ass-holes in the school were on the football team. They were the ones who played tricks on Freshmen, mocked people who were actually intelligent, and tried to get out of doing work (school work or otherwise), so I CONCLUDED that all football players were ass-holes. It may not have been literally true, but true enough for my purposes. If I met a football player who turned out not to be an asshole, I didn't change my general opinion, I just saw this one person as an exception.
BTW, I agree with you that school uniforms are a good idea. They eliminate one source of conflict and preoccupation. But teens will still act like teens, and no amount of teacher effort will change that.
