Poppy Moore, a wealthy but spoiled and rebellious girl from Malibu, California, damages and discards the belongings of her widowed father's new girlfriend, Rosemary. She grabs a handful of Rosemary's clothes and jumps into the ocean just as her furious father, Gerry, arrives. Gerry announces he will be sending Poppy to Abbey Mount, a boarding school in England, in the hopes that attending the school will turn her life around. Arriving at Abbey Mount, she is warmly greeted by the headmistress Mrs. Kingsley. Later she meets the head girl, Harriet Bentley, who is disgusted by her sense of entitlement. Poppy learns to her dismay that the bedrooms are communal and is introduced to Kate, Josie, Kiki and Jennifer, also known as "Drippy". She is initially hostile towards her new roommates upon meeting them, and her behaviour gets them into trouble when she insults the Matron, who confiscates their cellphones and gives them three weeks of detention.
When Kate asks Poppy about her mother, she reveals that she died in a car crash when Poppy was eleven years old. Kate lets Poppy use her actual cellphone to message her friend from back home, Ruby, who is secretly cheating with Poppy's boyfriend, Roddy. The girls decide to help Poppy get expelled from Abbey Mount by carrying out several pranks, which gradually brings them closer together. When none of their plans work, they decide to have Poppy seduce Mrs. Kingsley's son, Freddie, who is forbidden from fraternizing with the female students. After the girls go shopping at a charity shop, they take Poppy to a hair salon for a makeover, where hairdresser Mr. Christopher removes her blonde hair dye and reveals her natural brunette hair. At the school dance, Freddie rejects Harriet in favour of dancing with Poppy, much to Harriet's dismay. Poppy trips on the dancefloor and Freddie takes her outside for fresh air, where he asks her out on a date.
The next day, Poppy discovers her newfound talent for lacrosse and becomes the team's new captain. After several weeks of practicing and improving their skills, the team advances to the finals for the first time since 1976. Finding themselves falling in love with one another, Poppy and Freddie go for a drive in the countryside, where Freddie takes Poppy out for lunch and they kiss. Poppy returns to school only to find her roommates reading an email allegedly written by her, suggesting she was pretending to be friends with them so she could get herself expelled and return to Malibu. Poppy claims she did not write the emails, but the girls refuse to listen and walk out on her. When Freddie receives a similar email, Poppy tries to explain herself, but he slams the door in her face. With no one else to turn to, Poppy sneaks into the kitchen and calls Ruby, who accidentally reveals her affair with Roddy and hangs up on her.
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 back to her bedroom, but a few minutes later, Drippy sees the curtains engulfed in flames and is locked inside the freezer. When Poppy is awoken by the sound of an explosion, she looks out the window and sees the fire growing and immediately wakes the school. When everyone realizes Drippy is missing, Poppy, realizing she must be in the freezer due to seeing her do it before, runs into the burning school to rescue her. After the fire is extinguished, Freddie finds her lighter and gives it back to her, refusing to hear any explanation. Realizing she no longer wants to leave, Poppy writes an apology letter to Freddie and later goes to Mrs. Kingsley's office to take the blame for the fire. Afterwards, she finds a portrait of her late mother in the 1976 Abbey Mount lacrosse team and realizes her mother had attended the school.
While Poppy waits for the Honour Court to decide whether or not she should be expelled, Freddie finds her crying and becomes convinced the fire was an accident, and forgives her. Meanwhile, Drippy reads aloud Poppy's diary, revealing how much Poppy likes her roommates, and Kiki discovers that Harriet wrote the emails. At the hearing, Poppy gives her testimony and Harriet is determined to prove her guilt, while Poppy's roommates arrive and begin whispering to the other girls. When they all stand up in support of Poppy, Harriet bursts into a tirade and mentions Poppy's lighter. The girls realize that no lighter was ever mentioned and Mrs. Kingsley questions how Harriet knew about the lighter. When Poppy deduces that Harriet was responsible, Harriet confesses to restarting the fire after Poppy had put it out. Mrs. Kingsley summons Harriet to her office, ensuring her expulsion and exonerating Poppy in the process.
At the lacrosse finals, Gerry arrives and is taken aback by Poppy's dramatic change in personality and how much she resembles her mother. Abbey Mount wins the lacrosse championship and Poppy, who admits that her father did the right thing by enrolling her at the school, reconciles with him. The next morning, Harriet prepares to leave with her father, but not before her former friends toss her dead pheasants out the window and onto her. Several months later, Poppy and Freddie have resumed their relationship, and she invites her roommates to a pool party at her Malibu mansion. When Freddie notices Ruby trying to call Poppy, she ignores her, now well aware she is not a real friend, and the girls prepare to jump into the ocean.
Emma Roberts as Poppy Moore
Natasha Richardson as Mrs. Kingsley
Alex Pettyfer as Freddie Kingsley
Aidan Quinn as Gerry Moore
Georgia King as Harriet Bentley
Kimberley Nixon as Kate
Juno Temple as Jennifer "Drippy" Logan
Linzey Cocker as Josie
Sophie Wu as Kiki
Shirley Henderson as Matron
Shelby Young as Ruby
Johnny Pacar as Roddy
Daisy Donovan as Miss Sara Rees-Withers
Jason Watkins as Mr. Roger Nellist
Nick Frost as Mr. Christopher
Ruby Thomas as Jane
Eleanor Turner-Moss as Charlotte
Lexi Ainsworth as Molly Moore
Kelsey Sanders as Abbey Mount student
Daisy Bevan (uncredited)