K-On!'s story revolves around four Japanese high school girls who join Sakuragaoka Girl's High School's light music club to try to save it from being abolished. However, they are the only members of the club, and at first Yui Hirasawa, the main character, has no experience playing musical instruments or reading sheet music. Eventually, she learns how to be an excellent guitar player. Since then, Yui, bassist Mio Akiyama, drummer Ritsu Tainaka, and keyboardist Tsumugi Kotobuki spend their school days practicing, performing, or just hanging out together. The club is overseen by music teacher Sawako Yamanaka, who eventually becomes their homeroom teacher as well during their final year of high school. In their second year of high school, the club welcomes another guitarist, an underclassman named Azusa Nakano. After Azusa joins, they gain more structure and begin to practice more.
After their third year, Yui, Ritsu, Mio, and Tsumugi graduate and enroll into a university. It is there they join its light music club alongside three other students: Akira Wada, Ayame Yoshida, and Sachi Hayashi. Meanwhile, Azusa continues to run the high school light music club alongside Yui's sister, Ui, their classmate, Jun Suzuki, and new member Sumire Saitō.
Informations Source: Wikipedia