Dewey Finn (played by Jack Black) is a struggling musician who feels stuck in a rut. As his bandmates kick him out of their group, he receives a phone call intended for his roommate and best friend, Ned Schneebly (played by Mike White). The caller is Rosalie Mullins (played by Joan Cusack), principal of the prestigious Horace Green prep school, and she seeks a substitute teacher for a few weeks.
Desperate for money, Finn decides to impersonate Ned as a substitute teacher and take the job himself. However, he soon discovers that the students aren't interested in learning the subjects he has been assigned to teach. Instead, they are sapped of energy, confidence, and imagination.
Finn becomes determined to turn their academic lives around and inspire them through the power of rock 'n' roll. He secretly transforms the classroom into a makeshift music studio and trains his pupils to play various instruments and appreciate different musical styles. Thus, the titular "School of Rock" is born.
Finn coaches the students into forming a band and rehearses with them diligently. They help him to write and perform original songs that stand out in their school's Battle of the Bands competition. Meanwhile, he learns from the kids that he's much happier as a teacher than he ever was as a rockstar.
As Finn gets closer to his students, he discovers some of their private struggles and helps them cope with their issues. He also learns from Mullins that Ned has feelings for her, and tries to play matchmaker for his roommate by helping him reconnect with Mullins.
Mullins catches on to Finn's scheme and initially believes his teaching methods are inappropriate, but eventually warms up to him and his influence on the children. The film ends with the characters performing a triumphant concert and Finn being hailed as a hero by his students and colleagues.