Assassination Classroom (2012)

Blurb:
The students in Class 3-E of Kunugigaoka Junior High have a new teacher: an alien octopus with bizarre powers and unlimited strength, who’s just destroyed the moon and is threatening to destroy the earth - unless they can kill him first!

Meet the would-be assassins of class 3-E: Sugino, who let his grades slip and got kicked off the baseball team. Karma, who’s doing well in his classes but keeps getting suspended for fighting. And Okuda, who lacks both academic and social skills, yet excels at one subject: chemistry. Who has the best chance of winning that reward? Will the deed be accomplished through pity, brute force or poison...? And what chance does their teacher have of repairing his students' tattered self-esteem?

Source: www.anisearch.com/manga/9984