Assassination Classroom S0.5 SP Metting Time E0 (2014)

Koro-sensei reminisces about his first meeting with Karasuma, where they first butted heads and fought against each other, ultimately coming to respect one another. However, Karasuma eventually points out their actual meeting wasn't nearly as eventful.

Assassination Classroom S1 (2015)

When a mysterious creature chops the moon down to a permanent crescent, the students of class 3-E of Kunugigaoka Middle School find themselves confronted with an enormous task: assassinate the creature responsible for the disaster before Earth suffers a similar fate. However, the monster, dubbed Koro-sensei (the indestructible teacher), is able to fly at speeds of up to Mach 20, which he demonstrates freely, leaving any attempt to subdue him in his extraterrestrial dust. Furthermore, the misfits of 3-E soon find that the strange, tentacled beast is more than just indomitable—he is the best teacher they have ever had! Adapted from the humorous hit manga by Yuusei Matsui, Ansatsu Kyoushitsu tells the story of these junior high pupils as they polish their assassination skills and grow in order to stand strong against the oppressive school system, their own life problems, and one day, Koro-sensei.

Assassination Classroom M1 365 Day's' Time (2016)

A year can change a person's life forever. The 365 days Class 3-E of Kunugigaoka Junior High spent with their eccentric teacher, Koro-sensei, certainly did. Carrying the memories of that year close to their hearts, alumni Nagisa Shiota and Karma Akabane return to their former classroom to recall the events of that momentous time of their lives. Nagisa and Karma are reminded by the familiar rooms, desks, chalkboard, and the class album of the events that shaped them into "assassins" and prepared them for the real world. That is the legacy of their octopus-like teacher—who had introduced himself by threatening to destroy the world if they didn't kill him by the end of the school year—and the time spent in the bizarre yet exceptional "Assassination Classroom."

Assassination Classroom S2 (2016)

Returning from their summer vacation, the students of Class 3-E at the prestigious Kunugigaoka Middle School find themselves up against unbeatable odds. Faced with the possibility of world annihilation, the students must come up with increasingly elaborate and creative ways to kill their teacher, the cunning yet optimistic and helpful Koro-sensei.
However, eliminating Koro-sensei is not the only objective the students need to worry about. Gakuhou Asano, the academy's merciless and cruel principal, seeks to prevent Class 3-E's success by brainwashing his other hard-working pupils into ruthlessly competitive studying machines. Hostility begins to linger in the air as traitors and killers alike attempt to claim the bounty on Koro-sensei's head for themselves.
Nagisa Shiota, one of Class 3-E's most skilled assassins, finds himself in the middle of the conflict. While he works to maintain his academic standing and prevent the end of the world, domestic affairs jeopardize his place in Class 3-E. Together with his dedicated classmates, he must now face the threats head-on.

Assassination Classroom ONA Extracurricular Lesson [SIDE STORY] ONA] (2016)

Shorts of Ansatsu Kyoushitsu 2nd Season which adapt chapters of the manga which are not in the TV series.
The adapted chapters are 78 ("Takebayashi no Jikan") and 79 ("Shihaisha no Jikan, Ni Jikanme"), which are published in the ninth manga volume.