Thesis: The five main characters in John Knowles' A Separate Peace represent social stereotypes, according to some people.
In his book A Separate Peace, John Knowles represents jocks with Phineas, a character who believes that sports are the key to life. Phineas is more of a sportsman than a jock. Real jocks only care about winning, Phineas makes sure it's not possible for anyone to win or lose.
Chet Douglas is an exaggerated prep, just like Phineas is an exaggerated jock. He is obsessed with learning just for the sake of learning. No real hardcore prep thinks that way! Chet Douglas lives in his own educational world. He's so absorbed in this alternate reality in which Calculus has a justified existence that he forgets what the school is trying to teach him, and actually goes out and seeks more academia than what the school is already shoving down his and everyone else's throat. The standard prep is only concerned with being on the top of the Honor Roll, so that everybody's parents can marvel at how smart (s)he is.
There is one character that fits into no stereotype. "Leper" Lepillier is an individualist. Individualists are people who don't conform to social norms just for the sake of being accepted by others. Real individualists are not those people with blue and green hair you see on talk shows. Those people conform to a subculture, something that was less common during World War II. The real individualists of the world are quickly disappearing, as conformity becomes more popular. I haven't met any real individualists, so I can't say whether or not Knowles exaggerates Lepillier's lack of stereotype.
In modern society, there is pressure on individualists to conform to the most prominent subculture in the local area (I think). Those who fail to conform become outcasts, like the character Quackenbush. Outcasts are ridiculed so that they see themselves as inferior to everyone. In the book, Quackenbush tries desperately to find someone who he is not inferior to, and starts a fight with Gene. While outcasts are created in the same way as Quackenbush, he reacts much differently to being an outcast than most outcasts do. To the standard outcast, everyone is the target, eventually, not just a few inferior people.
Some people are self-obsessed. Gene would be a good example if he was real. Gene is overly obsessed with his own emotions, this is why we have to put up with all his garbage throughout the book. He is the type who would step over his own mother (or shake his best friend off of a tree) to get what he wants.
Since the time when this book takes place, new groups of people called subcultures have formed. Each subculture has its own social norms. They are like a society within a society, and segregate the people even more than these five basic types of people did back in Knowles' time.