The First Murder

"We're murderers." Mayu is an ordinary high schooler who goes to an all-girls school. She lives a fulfilling high school life with her friends, Oto and Yuu. However, one day, Mayu and Yuu witness Oto pushing her homeroom teacher over, who then falls unconscious. Oto says that it's just a temporary loss of consciousness, but...!? (Source: Young Jump, translated)