Ansatsu Kyoushitsu [Manga]

The story is about class 3-E of Kunugigaoka Middle School where every morning they greet their sensei with a massive firing squad. The sensei is a weird combination of a ALIEN and an octopus that moves at speeds of mach-20. It turns out this creature was responsible for the destruction of the moon, rendering it forever in a crescent shape. He has announced that he will destroy the world in 1 year. The creature will teach class 3-E how to assassinate him before the year is over