THE social, peer-group world of the American junior high school is a formative experience many adolescents find cruel. The group or club or team one belongs to may define or identify youngsters, often incorrectly, denying them a chance for full participation.Social cliques that exclude are set up around sports teams and cheerleading squads. The selection process for such groups can be arbitrary - often an exercise in social Darwinism. The better qualities of individual students are often tossed aside. Petty and cheap standards can hold sway: How pretty, how popular, how daring, how willful. No wonder so many students are turned off, and so many schools have a culture built on cynicism. But Plainfield Community Middle School, in Plainfield, Ind., has a different idea. As reported in the New York Times recently, Plainfield opened its cheerleading squad and sports teams up to anyone who wants to participate. And it turns out that many do. There are 72 cheerleaders this year, up from 60 last year. The football team numbers 69, and there are 64 swimmers, 65 wrestlers, and 149 members of the track team. No one gets cut. Such a program presents obvious logistical problems, and it can't be duplicated everywhere. Yet the idea challenges other schools to set up programs that have a generosity of spirit, offer more kids a chance, ease peer pressure, put social hierarchies in perspective, and help students and parents take the junior high world less seriously. A recent case of a Texas woman plotting murder to get her daughter on a cheerleading squad suggests tensions that must be dealt with. Of course, much can be said for fair, well-run traditional programs. Not making a team can be as educational as making one, or even more so. Certain talents ought to be allowed to develop and shine, not be drowned out by an imposed egalitarianism. Nor does one want to simply create a bigger clique. A bigger squad could create more parental pressure on kids to join. Maybe some shouldn't.