Ansatsu Kyoushitsu

Class 3-E sits at the bottom of Kunugigaoka Junior High, isolated from the main campus and treated as a warning to every other student. Then the class receives an impossible assignment: assassinate their new homeroom teacher before graduation, or he will destroy Earth just as he already destroyed most of the Moon.

The target, an extraordinarily fast yellow creature nicknamed Koro-sensei, is almost impossible to kill. Worse for the mission, he is also the most attentive teacher the rejected students have ever had. As Nagisa Shiota and his classmates learn assassination techniques, academics, and confidence from the very being they must murder, saving the world becomes inseparable from changing their own futures.