Assassination Classroom

The story revolves around the misfits of Class 3-E at Kunugigaoka Junior High School. They are given a seemingly impossible mission: assassinate their new homeroom teacher, an incredibly powerful, tentacled, octopus-like creature whom they name Koro-sensei (the unkillable teacher). Koro-sensei has already destroyed 70% of the moon and threatens to do the same to Earth within a year, unless his students can eliminate him first. The government offers a staggering reward of 10 billion yen to anyone who succeeds.