> In effect, attributing the worst case scenario to the general case.
I don't think that's a fair statement. If you watch "R-rated" depictions of brutal violence, the probability that you will be adversely psychologically (or socially) affected by those depictions is pretty close to zero. This is not because you have committed acts of brutal violence yourself, but rather because you are capable of dealing with such things in a mature and rational way. Similarly, the argument here is not that high-schoolers will be unaffected by this film because they have practised the behaviour depicted therein, but because they have dealt with it and understand the implications of such behaviour.
Language is a particularly important case because people outside the school system (and even some inside it) tend to believe that children are unspoilt bundles of verbal innocence, whereas in reality the sort of language practised on a day-to-day basis by your average 13-year-old is probably considerably more offensive than the language used in this film. It's ironic that a film like this one should be crushed by the very veil of ignorance which it attempts to lift - social norms change, and the MPAA can't expect to be able to stick its head in the sand and assume that a ratings system based on a fairly static set of "family values" will be relevant forever.