At its core Ansatsu Kyoushitsu (Assassination Classroom) is about assassinations and teaching and how such a relationship creates an uniquely optimium learning environment for the class focused on during the story. And yeah there’s tentacles.
The story is one involving an unusual super powerful being resembling that of an octopus who claims to have been the reason behind why the moon is missing a large chunk (70%) of itself, rendering it now into a shape of a crescent moon. This unusual being declares that within a years time he will destroy the Earth, just like how he destroyed the moon, but to give humanity hope, he gives them a chance by becoming the homeroom teacher for a specific class of junior high students known as “Class-E”. These students among learning the normal junior high school study curriculum must also polish their skills in assassination to carry out an assassin to kill him, their homeroom teacher, who they name Korosensei (unkillable teacher). The government fearing for what might happen in a years time, offers 10 billion yen to any of the students (or person) able to kill Korosensei.