Read manga: Ijimekaeshi. -Fukushuu No 31-

manga
Summary:

I was bullied by everyone, so in return I'm going to slaughter them all.Taking inspiration from the 31 days of the calendar, the uber popular girl group Calendrier was formed. Behind the bubbly appearance, in the shadows, there is one member that is bullied by the others on a daily basis. "If you can't smile, then neither will the fans."She is willing to do anything to regain the smile that was robbed by her group mates.With 30 members as her opponents, this is the story of the crazy number 31.