Elizabeth Halsey (Cameron Diaz) is a Chicago middle school teacher at the fictional John Adams Middle School who curses at her students, consumes lots of alcohol, smokes marijuana, and only shows movies while she sleeps through class. She plans to quit teaching and marry her wealthy fiancé, but when he dumps her, she must resume her job as a teacher. She tries to win over substitute teacher Scott Delacorte (Justin Timberlake), who is also wealthy. Amy Squirrel (Lucy Punch), a dedicated teacher and colleague of Elizabeth, also pursues Scott while the school's gym teacher, Russell Gettis (Jason Segel), makes advances on Elizabeth which she rejects.[3]



Elizabeth plans to get surgery to enlarge her breasts, believing she is being overlooked for women with larger chests. However, she cannot afford the $10,000 procedure. To make matters worse, Scott admits that he has a crush on Amy, only viewing Elizabeth as a friend. Elizabeth attempts to raise money for the surgery by participating in her 7th grade class car wash in provocative clothing and by manipulating parents to give her money for more school supplies and tutoring, but her efforts are not enough. Amy, acting on the growing resentment between them due to her pursuit of Scott and ignoring of school rules, attempts to warn the principal about Elizabeth's embezzlement scheme, but he dismisses her claims as groundless.



Elizabeth later learns that the teacher of the class with the highest state test scores will receive a $5,700 bonus. With this knowledge, Elizabeth decides to change her style of teaching, forcing the class to study intensely for the upcoming test. However, the change is not enough and the students have low scores on their quizzes, frustrating her even more. Meanwhile, she befriends Russell the gym teacher as Amy and Scott start dating. Elizabeth steals the state test answers by impersonating a journalist and seducing Carl Halabi (Thomas Lennon), who is in charge of the exam. Elizabeth drugs Carl in his office, and steals the test. A month later, Elizabeth wins the bonus and pays for the appointment to get her breasts enlarged.



When Elizabeth learns that Amy and Scott are chaperoning an upcoming field trip, she taints an apple with poison ivy and leaves it for Amy. Amy ends up with blisters covering her face and backs out of the trip. On the trip, Elizabeth seduces Scott. They engage in non-penetrative sex, and Elizabeth secretly calls Amy to ensure she knows. However, Scott's peculiar behavior starts annoying Elizabeth. Elizabeth later gives advice to one of her students (Matthew J. Evans) who has an unrequited crush on a superficial girl in class, which causes her to reflect on how she has been superficial as well.



After hearing Elizabeth and Scott having sex, Amy switches Elizabeth's desk with her own to trick the janitor into unlocking Elizabeth's sealed drawer. The evidence Amy finds leads her to suspect Elizabeth cheated on the state exam. Amy informs the principal and gets Carl to testify against her. However, Elizabeth took embarrassing photos of Carl while he was drugged and uses them to blackmail him to say she is innocent. Having noticed her desk was switched, Elizabeth informs the principal that some teachers in the school are doing drugs. When the police bring a sniffer dog to search the school, they find drugs in Amy's classroom, in Elizabeth's desk. Amy is moved to another school by the superintendent. Scott asks Elizabeth to start over, but Elizabeth rejects him in favor of a relationship with Russell.



When the new school year starts, Elizabeth is kinder to her co-workers, has started a relationship with Russell, and did not get the breast enlargement because she feels that she looks fine the way she is. Elizabeth also has a new position in the school as the new guidance counselor.