It's also kind of interesting that high school is basically the last time most people have to interact with anything like a random cross section of their peers. Maybe in big cities the public schools are more stratified by class and race, but in the small town I was from, basically everyone went to the same school. (Maybe some people were homeschooled, but I don't know how big a thing that was then.)

After high school, schools and workplaces tend to select for similar sorts of people.