One day, the moon exploded, leaving it in a perpetual crescent-moon shape. The creature who claims responsibility for the destruction also claims it will destroy the world in March of next year. With powers beyond human comprehension, no military can stand against it. Yet, for some reason, it chooses to take up occupancy as a teacher in the worst class of Kunugigaoka Middle School. The fate of the world rests in the hands of its students, as they have until graduation to figure out how to murder their teacher and collect ten billion yen for saving the world.

Source: ANN

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Other: Ansatsu Kyoushitsu
Sequel: Ansatsu Kyoushitsu (2016)
Alternative version: Koro-sensei Quest! (2016)
Side story: Gekijouban Ansatsu Kyoushitsu: Minna no Jikan