In a word, they need to be socialized. Of course, this is exactly what high school sets out to do. But high school socializes adolescents by forcing them to spend all their time, not with adults who offer examples of maturity, but with other adolescents. The only adults in grades 9-12 are teachers, whose role as disciplinarians casts them as adversaries rather than as role models. The result, predictably, is the warped culture that holds sway in the halls of most American high schools.
Adolescents are conformist, so the culture demands conformity.
Adolescents are vicious, so the culture is cruel beyond belief. Adolescents are insecure and anti-intellectual, so the culture despises academic achievement. When a student finally graduates out of this culture, he has undoubtedly gained a smattering of practical knowledge. But after four years in a shallow, conformist world, he is no closer to being an adult, really, than when he entered high school in the first place. But it's so important for kids to spend time with their peers, the objectors will bleat.