This film provides examples of the following tropes:
Adults Are Useless: Averted with Ms. Norbury, who proves to be very understanding of what Cady and the other girls are going through. However, played straight with Cady's parents and especially Regina's mother.
Adult Fear: Regina's mother watches her daughter get hit by a school bus.
Alpha Bitch: Regina and, later, Cady. The whole movie is arguably a Deconstructive Parody of this trope, demonstrating how a girl can be so simultaneously loved, feared, and hated by the rest of the student body.
Ambiguously Gay: Regina claims Janice Ian is a lesbian, though she's actually Lebanese. Her outfits reinforce the stereotype, but by the end of the film it's obviously not true.
An Aesop: Be yourself (of course!). Also that judging and antagonizing each other doesn't really benefit anyone.
Artistic License - Geography: In one scene, the characters go to Old Orchard Mall (technically "Westfield Old Orchard"), a Chicago-area mall known for being a large, outdoor mall. Its stand-in was Sherway Gardens in Etobicoke, Ontario, a more conventional indoor mall.
Artistic License - Medicine: There is little chance someone who was hit by a bus and injured badly enough to need a halo (meaning, basically, that she damaged part of her neck) would ever be cleared to play a sport as physical as lacrosse, let alone in a matter of a few months.
Backhanded Apology: When choosing a fault to apologize for before falling backwards and letting others catch you, it is not a good idea to apologize for being so popular it makes everyone else jealous.
Janis also does this with Regina when she "apologizes" for helping Cady in sabotaging her and says she did it because "I guess I've got a BIG, LESBIANCRUSH ON YOU!"
Badass Teacher: Principal Duvall. Doesn't do anything typically badass during the film, but sometimes sheer presence is enough: looking at the man, especially when he's giving a stern lecture whilst wielding a baseball bat, just tell me you wouldn't a) hang on to his every word or b) not want to take him on in a fight.
Based on an Advice Book: Queen Bees & Wannabes by Rosalind Wiseman, written as a guide for parents with teenage daughters to help them understand the social structures and viciousness of high school life.
All of the Plastics' rules about how they dress, act, etc. come almost word-for-word from one girl's actual testimonial about her clique in the book. Think about that for a second.
Cady:[narrating] I know it might look like I'd become a bitch, but that's only because I was acting like a bitch.
Bilingual Bonus: Trang Pak and Sun Jin Dinh's dialogue becomes a lot funnier if you speak Vietnamese. Also, at the end of the movie, when Gretchen is brown-nosing to the Cool Asians in Oriental-sounding gibberish, one of them says (in confused and unsubtitled Vietnamese), "What?"
Another one of sorts: "Regina" is the Latin word for "Queen". Makes sense, no?
Bowdlerization: Tina Fey's original script was a raunchy, R-rated sex comedy in the vein of Porky's and American Pie. It featured much stronger sexual references — one of the characters admits to masturbating with a hot dog note In the final version, "masturbated" was dubbed over with "made out" — and anyone who knows how to lip-read can tell what the original line was supposed to be., and the scene where Gretchen is caught kissing a boy in the bathroom was supposed to be her performing oral sex on him. Regina and Karen were to have topless scenes, and their Halloween costumes were also supposed to be skimpier than what was shown in the final film, amounting to pretty much what Cady describes: lingerie plus animal ears. There was also a subplot involving an ecstasy-popping Club Kid named Barry. These were changed when Lindsay Lohan was hired to play the lead, as she was trying to keep a squeaky-clean image at the time.note Oh, if they only knew what would happen to her years after the movie came out...
One could argue this is for the better, as it makes Mean Girls more accessible to the teen demographic that can relate to the characters, and also makes it stand out from the pack of movies in the genre.
This trope also applies whenever the movie is aired on a "family" channel such as ABC Family. All the strange and varyingly clumsy ways they try to cover up or eliminate anything objectionable would make for a great drinking game (if there wasn't the dangerous possibility of the players dying of alcohol poisoning).
Face-Heel Turn: Over the course of the film, the friendly and good-hearted Cady usurps Regina's position and becomes the mean-spirited Alpha Bitch of the school. She goes through a subsequent Heel-Face Turn when she's confronted with her actions.
Heel-Face Turn: Cady at Spring Fling, and all the Plastics at the end.
Arguably before that, when Cady takes the blame for the Burn Book.
Heel Realization: Cady's response when Janis points out that she's become just as bad as the Plastics is to cry.
Happens a second time when she squares off against the remarkably unstylish girl in the rival math club. Cady realizes that making fun of her appearance and horrendous fashion sense would not win the competition, that calling someone else ugly won't make you more beautiful, etc.
Hidden Depths: Gretchen confesses to Cady that she's secretly miserable as Regina's friend and has to pretend to like and not like certain things to get Regina's approval.
Important Haircut: Off screen. After Regina spread the rumor in middle school that Janis was a lesbian, Janis dropped out of school for the rest of the year; when she returned in the fall, "all her hair was cut off, and she was totally weird."
Karma Houdini: Janis, who, despite influencing and encouraging Cady to join the Plastics specifically to damage Regina, gets zero comeuppance when she reveals it to the entire crowd of girls following the revelation of the Burn Book. In fact, she gets applauded for it! Cady, in the meantime, is treated as a bitch by everyone because of what she's done, even though Janis admitted that she was the mastermind behind all of it.
Kissing Cousins: At the Halloween party, Karen talks about how hot her cousin is. When confronted, she says, "Yeah, but he's my first cousin", apparently believing this to be a more distant relation than a "cousin".
Look Both Ways: Regina gets hit by a bus for not doing this. (She gets better.)Foreshadowed with Cady narrowly missing one on her first day.
Lucky Translation: Karen goes to the Halloween party dressed as a sexy mouse. In the Italian version, when asked what she is dressed as, she answers "Sono una topa." "Topa" means "female mouse," but is also a slang word for female genitalia and a rude compliment to a pretty girl.
Gretchen isn't all that malicious either, just desperately insecure and hungry for attention, which makes her extremely prone to gossip and over dramatize things ("Pusher? Like a drug pusher?"). Makes more sense if you consider that in the original script she was described as ugly.
Memetic Badass: He doesn't get any lines, but he still gets a lot of play on the internet, "Four for you Glen Coco! You go Glen Coco!"
Misplaced Wildlife: During Cady's party, Aaron sees a picture of Cady back in Africa riding an elephant. An Asian elephant.
Never Trust a Trailer: Averted for the most part, the trailer generally gives an accurate description of the movie, but it notably switches Janis and Damien's descriptions of Gretchen and Regina.
No Dress Code: The plastics get away with performing in a school pageant wearing sexy Santa outfits that are closer to lingerie than costumes. Also applies when Regina accidentally starts the trend of wearing shirts with holes cut over the breasts.
No Social Skills: Up until she came to the school, Cady lived in Africa and was home schooled, thus winding up with absolutely no clue about how things worked in "Girl World." Her parents appear to be clueless every time they appear:
Not So Different: Though Janis wants revenge on Regina, she's as spiteful and manipulative as she is when it comes to Cady. Janis even admits it, stating that she knows she's as mean as Regina, while Cady pretends that she isn't.
Pair the Spares: Mocked. Janis and Damian kiss at the dance for a second, then quickly separate from each other in disgust.
But then arguably played straight when she hooks up with Kevin at the end.
Playing Gertrude: 32-year-old Amy Poehler playing the mom of Regina, despite being just five years older than Rachel McAdams. Tina Fey even mentions this in one of the featurettes. Justified, in that a) she's Regina's stepmother, and was probably picked up as a young trophy wife, and b) it's implied that she has had excessive plastic surgery, her "hard-as-rocks" boob job being just the tip of the iceberg.
Popular Is Dumb: Karen obviously, and arguably Gretchen and Regina who seem somewhat unastute. Regina, despite being able to manipulate most of the female half of the school into becoming her pawns, frequently shows herself to be rather Book Dumb.
Professional Butt-Kisser: Gretchen. At the end of the film, Gretchen joins the Cool Asians now that the Plastics have broken up, and attaches herself to Trang Pak the way she did Regina. Her last scene has her brown-nosing in Vietnamese gibberish, with her hair straightened to look more like her Asian friends.
Rich in Dollars, Poor in Sense: Regina in a specific example. She could rattle off several mostly-illegal weight loss drugs (most of which either were expensive before being banned or would be expensive to ship from countries where they're still legal). Yet, she didn't know enough about health and nutrition to know whether or not butter is a carb.
Possibly just a commentary on how American teenage girls obsess over getting thin the fast and unhealthy way rather than via actual healthy living.
In the original script, the Sound Effect Bleep was this, used whenever a character dropped an F-bomb.
Satellite Love Interest: Aaron. All we know about him is that he's cute, sucks at math, used to date Regina, and "all he cares about is school and his mom and his friends". He gets fleshed out a little more in the original screenplay, which has a scene with him and Cady where he does the laundry, and explains that he helps out because it's just him and his mom.
Sequel Hook: Averted. The sequel has no recurring characters aside from Principal Duvall. It might appear that the "Junior Plastics" seen at the end are this, but the sequel focuses on a completely different cast of characters.
Sexy Santa Dress: The skimpy dresses that the girls wear for a Christmas pageant.
Stepford Smiler: Gretchen (the "one small step from a complete nervous breakdown" version, rather than the "empty inside" version).
Stripperiffic: Lampshaded at the Halloween party, where Cady comments on how in Girl World, Halloween is the one night of the year a girl can dress like a total slut and no one can say anything. Gretchen wears a cat costume made of skintight leather, Karen wears a skimpy, cleavage-revealing teddy (only the ears give it away as a costume - "I'm a mouse. Duh."), while Regina wears a "rabbit" costume that's nothing more than a Playboy Bunny suit. Cady, not knowing this and thinking that Halloween costumes are meant to be scary, wears pale makeup, novelty rubber teeth, and a Blood-Splattered Wedding Dress ("an ex-wife").
Uncanny Valley: In-Universe. Cady finds Mrs. George to fall deep into the Valley, thanks to her plastic surgery. The sight of her fake breasts approaching Cady is met with Psycho Strings, and Cady is clearly unnerved by her lack of reaction to her little chihuahua gnawing on her tit.
Principal Duvall: I don't care how long it takes. I will keep you here all night.
Joan the Secretary: We can't keep them past four
Principal Duvall: I will you keep here until four.
Wardrobe Wound: In a Deleted Scene, Cady tries to give Regina one by having a friend rig her cafeteria table so that it tips and spills food into her lap when she sits down. However, a member of Regina's Girl Posse sits where Regina was supposed to, thus becoming the prank's victim.
Weight Woe: Regina's "killer bod" is one of the items that comes under attack in Janis' plot. The means? Protein-heavy granola bars that are supposed to help athletes and malnourished children gain weight, passed to Regina as if they're diet bars. Despite Regina's weight gain being a significant plot point, the actress' size doesn't change in the slightest.
Women's Mysteries: The principal attempts to get the girls talking about about any "ladies' problems" they might have (referring to disagreements with each other. When the first one he singles out predictably starts going into Too Much Information territory he swiftly subs in Miss Norbury, saying "OK, I can't do this."
You Are the New Trend: Regina. One scene has Janis cutting nipple holes in Regina's shirt in the locker room to try and sabotage her wardrobe. Regina wears the top anyway and the next day, every girl has the exact same nipple holes in her shirt.
The sequel provides examples of the following tropes:
And later on E! under their "Movies We Love" banner. . .
Broken Aesop: The moral of the story seems to be "don't be a follower," but in the end, Abby decides to attend Carnegie-Melon instead of her original goal of NYU just to stick with Jo, who in turn based her college decision solely on it being the school her dead mother went to.
Chekhov's Gun: The paranoid neighbor. Specifically his security cameras.
Enemy Mine: In the climax, Jo's former friends decide they hate Mandi more than they hate her and agree to help her.
Step Sibling Rivalry: Mandi and Tyler, though oddly the dislike comes entirely from his side and Mandi, despite being a bitch to everyone else seems to have some sisterly feeling for him, even declaring him off limits to Chastity and calling his father (her stepfather) "our Dad". He's also the only character she's never directly mean to.
Shout-Out: Why does it feel like the football game between the Plastics and the Anti-Plastics is an homage to The Longest Yard (of all things)?
What the Hell, Hero?: When Abby finds out that Jo accepted money to be her friend, she takes it about as well as one would expect.
You Keep Using That Word: Badass is frequently used to describe Jo but the only really badass thing she does is drive her vespa into the school.
I would say that tackling Mandi and holding her down is pretty badass