Elizabeth Halsey (Cameron Diaz
) is a poor Chicago-area teacher who curses at her students, drinks heavily, and takes copious amounts of drugs. She plans to marry her wealthy fiance, but when he dumps her, she tries to win over substitute teacher Scott Delacorte (Justin Timberlake
), also wealthy. Their colleague, Amy Squirrell (Lucy Punch
) is also pursuing Scott, while phys ed teacher Russell Gettis (Jason Segel
) is making advances on Elizabeth.
Halsey had planned on getting breasts implants in order to impress Delacorte, but cannot afford the operation. To make matters worse for Halsey, Delacorte begins to have feelings for Squirrell and only views Halsey as a friend. She attempts to raise money for the surgery by participating in her 7th-grade class car wash and by manipulating parents to give her money for more school supplies. Her efforts, however, are to no avail.
She later learns of a bonus for the teacher whose class earns the highest state scores. Halsey decides to change her style of teaching, going from a teacher who only showed movies to one who makes her students study vigorously for the approaching exam. Alas, the change in her teaching style and her efforts to prepare her students is not enough, as they score low on the quizzes she gives. Meanwhile she begins a friendship with Gettis, the gym teacher, and Squirrell and Delacorte begin dating.
Halsey then decides to steal the state exam by posing as a journalist reporting on the exam's racial bias. When the rector she is interviewing, explains he cannot give her the exam due to protocol, she resorts to seducing and drugging him in his office and stealing it. She also takes nude pictures of him as insurance in case she gets caught. A month later Halsey wins the bonus and pays for the appointment to get her breasts enlarged.
When she hears about a field trip her class is taking, which Delacorte and Squirrell will chaperone, she puts poison ivy on an apple and leaves it on Squirrell's desk for her to eat, causing her to not be able to go on the field trip, and Halsey replaces her. On the field trip she seduces Delacorte and they have sex, which Squirrell finds out through Halsey leaving a 'revealing' voice message on her cell phone. She also advises one of her students on letting go of a crush who doesn't like him because she is too superficial, causing her to reflect for a moment on how she has been superficial as well with her love interests.
After hearing Halsey's message, Squirrell switches her desk with Halsey's in order to get a someone to unlock her desk for her to find evidence that Halsey won the bonus unfairly. Squirrell informs the principal and a meeting is set up for Halsey discuss the matter with the principal and the superintendent. She gets away with her mischief through blackmail and informs them that some teachers in the school are doing drugs. When the police search the school, they find drugs on Halsey's desk (which is now in Squirrell's classroom), thus throwing suspicion onto Squirrel, who is moved to another school by the superintendent.
With Squirrell gone, Delacorte tries to get sparks going with Halsey. She, however, turns him down due to her realizing that she has a better thing going with Gettis, who is more down to earth.
The next school year it is shown that Halsey didn't get the breast enlargement because she feels she doesn't need it, and she looks fine the way she is. She also seems kinder to her coworker, and has begun a relationship with Gettis. As well, instead of returning to her English classroom, Halsey is now the school's new Guidance Counselor.