Assassination Classroom Vol 1

Assassination Classroom by Yusei Matsui. A Shonen manga rated Older Teen, published in English by Viz Media.

A smiling, tentacled creature who blew up most of the moon promises to destroy Earth in one year, unless the misfit students of Class 3-E can assassinate him first. The catch: he's also their homeroom teacher, and the best one they've ever had. A hilarious, surprisingly heartfelt action comedy.