Writer Tina Fey has said her original draft for the film was partially an homage to teen sex comedies of the 1980s, including "wall-to-wall titties" because several of the female leads had nude scenes. After Lindsay Lohan was cast the studio decided to make the movie more kid-friendly and removed the scenes containing nudity and sex.

In the US, the film was originally given an R rating, due to its sexual material. Director Mark Waters edited down the content, so the MPAA would give the film a PG-13. Examples of material that was changed in order to obtain the PG-13 rating include:

* Cady originally catches Gretchen performing oral sex on Jason in her bathroom at her party, instead of finding them making out as is seen in the final film.
* Jason originally asks Cady if her "cherry is popped", instead of asking her if her "muffin is buttered", as is heard in the final film. The line was dubbed over the original footage, so that Jason still asks "Is your cherry popped?", but is heard to ask "Is your muffin buttered?"
* Originally, The Burn Book states that Amber Delessio masturbated with a hot dog, instead of merely having made out with a hot dog. Watching Delessio's mouth carefully as she reads the page in the book reveals that the line was dubbed over the original one.
* The original script called for Regina to appear topless. In order to seduce Aaron away from Cady, she removed her top in a car and allowed him to feel her bare breasts. Regina made sure Cady would appear at the right time so she would witness the act.
* Cut after the first draft was a scene where Karen strips off her shirt and bra to go topless on the school's live newscast. Since she believes her breasts can predict the weather, she landed the job as the newscast's weather girl. Karen thought her breasts detected a tornado nearby and to get a more accurate prediction she ripped off her top, exposing herself to the entire school.

The final film received a PG-13 rating from the MPAA.