Assassination Classroom (Ansatsu Kyoshitsu the Movie)

An enigmatic octopus-shaped creature suddenly shows up as the homeroom teacher of Class 3-E, the worst class in the prestigious Kunugigaoka Junior High School. The government assigns the students of Class E with a secret mission to assassinate the mystery creature, aka "Korosensei," who is threatening to destroy Earth next March. The reward is an astounding 10 billion yen to whoever succeeds! Will the students be able to accomplish their mission before they graduate?