Assassination Classroom

is an ongoing Japanese manga written and illustrated by Yūsei Matsui that was serialized on the Weekly Shōnen Jump magazine starting in issue 31, released July 2012.

Plot
A strange new teacher is introduced to class 3-E at Kunugigaoka Junior High School. The teacher is a round-headed, tentacled alien capable of moving at speeds of up to Mach 20 and whose color changes depending on his mood. Prior to introducing himself to his new class, he managed to carve out a large chunk of the moon to become a permanent crescent moon. In a year, he plots to destroy the earth, but decides to teach a junior high school class in the meantime for reasons unknown. With this strange alien having effectively taken over the world, the government reluctantly complies with his request on the condition that he does not harm his students.

Affectionately, the students of class 3-E call him Korosensei, the unkillable sensei. These same students greet him everyday with an assassination attempt in their journey to become professional hitmen, to save the world by eventually murdering their own teacher, and to earn 10 billion yen from the government.

Official

 * Japanese Shonen Jump Ansatsu Kyōshitsu website