Fukushuu Kyoushitsu

Fujisawa Ayana was horribly bullied at school. At first, she tried to defend herself, but she soon learned to just stay quiet and wait for the pain to end. However, the bullying escalated one day, when someone pushed her into traffic. At the hospital, as she recovered, she suddenly realized that even if she didn't fight back, her classmates were probably going to eventually kill her.

From this realization, she soon decided that the only path left was for her to take revenge. Her new plan included the students who bullied her, those who stood aside and laughed, and those who did nothing to make it stop. She began gathering notes on her classmates, planning exactly what she could do to make each of their lives collapse. While some of the students really seemed to deserve their fate, her plan also included classmates that she used to count as close friends...

(Source: MangaHelpers)