In a wild prank with her friends, Poppy Moore (Emma Roberts), an affluent California teenager living in Malibu, ruins all her father’s (Aidan Quinn) girlfriend’s belongings. When her father angrily arrives, he tells her that he is sending her to a boarding school in England.
Arriving at the school, called Abbey Mount School, Poppy meets the headmistress, Mrs. Kingsley (Natasha Richardson), head girl, Harriet Bentley (Georgia King), and her ‘big sister’, Kate (Kimberley Nixon). She is miserable in her new life as she does not fit in and has no friends. After two weeks, Poppy sneaks out after lights-out and writes an email to Ruby, her best friend in Malibu, complaining about her new school and classmates. She sees one of her roommates, Drippy (Juno Temple) walking through the hall, and follows her to the school’s freezer, where Drippy is indulging her ice cream craving.
Disgusted, Poppy walks back. As she is, however, there is a fire drill, and she sneaks out the window to avoid being caught in the halls without permission. She ends up in the bathroom of an unknown man, who she assumes is a teacher, and tells him her name and is directed on how to get out. During a fight that ensued with Harriet, she learns that the person was actually Freddie Kingsley (Alex Pettyfer), the headmistress’s son, and Harriet becomes embarrassed and angry when Poppy exposes her crush on Freddie.
After getting dirty water on her by Harriet’s “maid” (really an intimidated Year 7), Poppy and Kate become friends. Revealing that the girls’ old cell phones were decoys meant to be confiscated, Kate lets Poppy use hers to call Ruby. When Ruby sees Poppy’s message, she is disgusted at Poppy’s appearance and it is revealed that Poppy’s boyfriend, Roddy, has been cheating on her with Ruby. After Poppy reveals that her mother died several years ago, her roommates decide to help her get expelled. Together they carry out several pranks which gradually brings them closer together. When none of their plans work, they decide to seduce Freddie, who is strictly forbidden to fraternize with any of the girls. After dancing with Freddie at the school dance, Poppy trips and hits her head which leads Freddie to take the concussed Poppy outside for air where he attempts to kiss her, but is caught by Harriet’s followers. Before she leaves, however, he asks her out on a date. The next day, after being scolded by Mrs. Kingsley for her behavior and drinking the previous night, Poppy discovers her talent for lacrosse, whips the school’s team into shape, and gets them into the finals (to the annoyance of Harriet, who was the former captain of the team). On the date, Poppy finds herself falling for Freddie. They kiss before Poppy returns to school.
In her room, Poppy finds her roommates reading an email supposedly sent by Poppy, stating that she was only pretending to be friends with them so they would help her get expelled. She then finds out that Freddie has been given a similar email and is thereafter ignored by her friends. With no one else to turn to, Poppy sneaks down to the cook’s room and calls Ruby, who accidentally reveals how much she dislikes Poppy and that she is seeing Poppy’s boyfriend.
Feeling even more alone, Poppy starts playing with her lighter and accidentally sets a curtain on fire. Hearing footsteps, she quickly puts out the fire and runs off. When she has returned to her room and in bed she has a terrible dream that the school actually is on fire. Horrified, she looks out her window and is shocked to see a real fire she then, feeling responsible, wakes up the school.
When they find that Drippy is missing, Poppy runs into the burning school to rescue her in the freezer (indulging her ice cream craving). After the fire is put out, Freddie finds her lighter and gives it back to her, refusing to listen to what happened and assuming that Poppy was the one who lit the building on fire.
While Poppy waits for the Honour Court to decide whether or not she would be expelled, Freddie finds her crying and becomes convinced that the fire was an accident. During the hearing, Poppy tells her story, and at the same time, her roommates find out that Poppy did not send the emails. Going to the court, they stand up for Poppy and Harriet accidentally confesses to restarting the fire after Poppy had put it out.
The movie is left off at the lacrosse finals. Poppy’s father comes to the game and is shocked by Poppy’s dramatic change in appearance and how much she looks like her mother, who was also captain of the lacrosse team at Abbey Mount. Abbey Mount wins the lacrosse finals and Harriet is expelled.