Koi ni Koisuru Koibito no Kankei

Asako Ichijou, the student council president at a prestigious girls' high school, has been racking her brain about how to discipline the unruly but popular Nagi Tenma. That's the sort of person Asako is, until Nagi discovers Asako has broken school rules, too, by bringing shoujo manga to school... Asako pleads with Nagi to keep it a secret, and Nagi agrees, but with the condition, "Become my lover." (Source: Takeshobo, translated) Note: Includes three extra chapters.