Anyone who goes against the plastics has their own label that they have to deal with. There are the jocks, the nerds, the goth kids, and so on. No one is just simply a teenager. And, when it comes down to it, the plastics end up turning on each other and exposing flaws that each member has in order to retain popularity and be the one left "on top".
What's the point of all this nonsense, you ask? I think that it's pretty clear that the whole story line is a hyperbole in itself. It exaggerates the stereotypes of high school, emphasizing the bad parts primarily. Doing so in a friendly manner allows the audience to understand the message that the creators were trying to convey while not getting offended or uncomfortable.
The movie is able to touch upon some very important topics even if it does not seem like it right off the bat. It teaches its audience about unnecessary social hierarchies that are sometimes created in high school. All that this creates are students who do not feel as though they can be themselves and people fighting to put others down so that they themselves can rise up. It also conveys the message that high school is a tricky time and that being a teenager is hard enough. There is no need to get involved in drama or other bad situations, since all that does is take away from the high school experience.
By over exaggerating high school stereotypes, the movie Mean Girls is able to emphasize important messages in a way that is easy to understand and talk about.