No Vacancy, a rock band, performs at a nightclub three weeks before auditioning for Battle of the Bands. Guitarist Dewey Finn creates on-stage antics, and a stage dive that abruptly ends the performance when the crowd doesn’t catch him. The next morning, Dewey awakes in the apartment he lives in with Ned Schneebly and Ned’s girlfriend Patty Di Marco. They inform him he must make up for his share of the rent, which is four months overdue, or move out. When Dewey meets No Vacancy at a rehearsal session, he finds out that he has been replaced by another guitarist named Spider. Later, while trying to sell some of his instruments for money, Dewey answers a phone call from Rosalie Mullins, the principal of the Horace Green Prep School, inquiring for Ned about a short-term position as a substitute teacher. Desperate, Dewey impersonates Ned and is hired. On his first day at the school, Dewey adopts the name “Mr. S” (as he cannot spell “Schneebly”) and spends his first day behaving erratically, confusing the class about lunch and giving them recess, much to their delight.
The next day, Dewey overhears the kids in music class and devises a plan to form them into a band to audition for Battle of the Bands. He casts Zack Mooneyham as lead guitarist, Freddy Jones as drummer, cello player Katie on bass, Lawrence on keyboard, and himself as lead vocalist and rhythm guitarist. He assigns the rest for backup singers, groupies and roadies, with Summer Hathaway as manager. The project eradicates teaching but helps the students embrace their talent. He reassures Summer, who is initially casted as groupie until researching about them on the Internet, Lawrence, worried of his social image, Zack, whose overbearing father disapproves of rock, and Tomika, not good enough to be a roadie and then worried to audition for being overweight despite having an amazing voice. During one lesson, he teaches the kids that rock and roll is the way to stick it to the man. Groupies Michelle and Eleni, with Summer’s approval, pitch the band name “The School of Rock.”
Two weeks later, Dewey sneaks the band members out to audition, while the rest of the class stays to maintain cover. When Freddy wanders off, Dewey retrieves him but the group is rejected because the bill is full. Summer leads the band to fake a terminal illness, successfully auditioning. The next day, Rosalie decides to check on Dewey’s teaching progress, forcing Dewey to teach the actual material. She explains that a parents’ night will take place the day before Battle of the Bands.
Dewey is finally exposed when Ned receives a paycheck from the school. Dewey confesses and begs Ned not to tell Patty, but she manages to reveal the truth out of Ned, and calls the police on Dewey. During the parents’ meeting, the parents question what Dewey was teaching until Ned, Patty, Rosalie, and the police confront him. Dewey reveals his real identity and is fired. Back home, Dewey admits what he did was wrong and giving up music isn’t easy for him. Ned admits it wasn’t easy for him to do so either, but that he had to know when to quit.