Actually most of them are black. The white teachers my wife had under her (she supervised about 100, total) were stumped at this phenomenon until black teachers explained it to them. They honestly had a hard time understanding it, even after it was pointed out to them by those who lived right in it every day.
Your own description of the positions your acquaintances hold demonstrates what I am saying. The majority of kids who give in to this peer pressure are never going to know what it is to associate with the people you spoke of. They are already consigning themselves to be on the fringe, and future welfare recipients, and they won't even be considered for higher positions by employers because they are shooting themselves in the foot, academically speaking, at this very crucial stage of their lives.
Stereotypes are not just fabrications of prejudiced minds... they truly DO come right from the observation of everyday life.