Fey wrote “Mean Girls,” too, and the comedic DNA that would make “30 Rock” so good when it debuted two years later is all over the script. There are jokes about the inherent hilariousness of German, the desperation of adults who want to be cool and the terrifying power of socially brilliant teenage girls. “Mean Girls” asks a question that “30 Rock” would pose, too: Why are the incentives stacked to drive girls and women away from their authentic selves?

Ms. Norbury, who mentors Cady even after her student is incredibly cruel to her, is an uncomfortable reminder that those problems do not really end in high school. Her recovery from her divorce and her growing confidence as a teacher and as a person may happen at the periphery of “Mean Girls,” but Fey’s performance is one of the reasons to return to the movie long after most of us have gotten over being victimized by the Plastics in our lives.

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When Cady, and we, meet Ms. Norbury, she is soaked in coffee and embarrassed in front of her cute principal, Mr. Duvall (Tim Meadows). “How was your summer,” Mr. Duvall wants to know. “I got divorced,” Ms. Norbury tells him. “My carpal tunnel came back,” Mr. Duvall offers. “I win!” Ms. Norbury tells him.

While Cady grows to like her perceptive teacher, who can sling a zinger with the best them, she is reluctant to embrace Ms. Norbury as a role model, especially when the siren song of the Plastics, the clique of popular girls who have embraced Cady, is tempting her away.

“Cady, I know that having a boyfriend may seem like the most important thing in the world. But you don’t have to dumb yourself down to get guys to like you,” Ms. Norbury tells Cady, after her promising student starts to fake bad grades in her class to attract a boy’s attention. She has zeroed in on what is happening to Cady, but what can Ms. Norbury offer Cady as a reward for taking her advice? “I’m divorced. I’m broke from getting divorced. The only guy who ever calls my house is Randy from Chase Visa,” Ms. Norbury admits. “And you know why? It’s because I’m a pusher. I pushed my husband into law school, that was a bust. I pushed myself into working three jobs, and now I’m going to push you.”

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Cady learns over the course of “Mean Girls” that giving up on your priorities and preferences to be liked carries just as high a price as the one Ms. Norbury has paid.

She blows up her relationship with her parents — the rare adults in a teen movie to actually be terrific and caring (if a bit unprepared for the pressures of teen culture). She alienates her friends Janis (Lizzy Caplan) and Damian (Daniel Franzese). She walks away from the Mathletes and a potential friendship with Kevin Gnapoor (Rajiv Surendra), a confident, funny boy whose fondness for math does not make him any less of a player in the high school dating scene.

Cady’s epiphany that there is another way to be happy in high school, and Ms. Norbury’s realization that her convictions have been worth it, come about simultaneously. After a fight between Cady and Regina radiates outward to consume all the girls in their graduating class, Mr. Duvall taps Ms. Norbury to help sort things out.

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“Ms. Norbury, you’re a successful, intelligent, caring, graceful woman,” Duvall tells her. “I am?” Ms. Norbury asks, taken aback. But she steps up to the task and ends up radically reshaping the dynamic between the girls, who have learned to keep secrets from each other, rather than working out their issues, and to treat each other as competition for boys, rather than as collaborators in sports, math competitions and art projects. Some of this transition might have happened anyway — “Mean Girls” cannily upsets the high school narrative by noting that cliques tend to break down as graduation approaches — but Ms. Norbury gets to be the agent of change at Northshore High.

By the time the school year ends, Cady has both rejoined the Mathletes and been elected Spring Fling Queen, bridging the divide that everyone told her was unbridgeable. When Cady snaps her plastic crown into pieces, distributing bits of its sparkle to her rivals and tossing them into the crowd at the dance, saying, “Share it,” the camera cuts to Ms. Norbury, smiling in a sea of teenage girls.