Hey people, I'm trying to find a manga that I read a while ago but I can't remember the title. It's been almost a year and I only read the first chapter. It's from the perspective of a highschool boy who's older sister works as a teacher at the school. Every seems normal, but one day he sees some kid getting bullied by some teenagers and steps in to help. Later on he runs into those same kids on the school roof. I don't remember what happens next, but something happens to the teenagers and they all start commiting suicide. Leaving the main character on the roof alone. That's all I really remember and it would be really helpful if someone knew what it was called.