Ansatsu kyôshitsu

The Earth's moon is suddenly 70% smaller and permanently crescent shaped by a devastating and unknown force. For the students of E Class, Kunugigaoka Junior High School, the being responsible is known, as their teacher. The strange and very supportive entity has given the Japanese government a challenge to save the Earth from destruction. It wants to be the teacher of the class with the lowest expectations for success, in anything. The students, armed with special weapons that only harm their teacher, must find a way to kill him before the end of the school year. If not, summer break will be canceled by the end of the world.