Assassination Classroom (2015 anime)

A 2015 television series based on the Shonen Jump manga Assassination Classroom.

Koro-sensei is a dangerous alien that has promised to destroy the world in one year. The governments of the world frustratingly tried to kill him but their methods failed again and again. But the creature possess a weird desire: to be a high school teacher to a class of students in Japan. The japanese government agreed under the conditions he shall never harm his students to keep his position as teacher.

They also put a 10 billion of yen bounty on him for whoever kills him, student or not. This will be a dangerous school year...for the teacher.

The first season of the show covers the manga's first 73 chapters in 22 episodes. The second season covered the remaining 107 chapters in 25 episodes.

Both seasons were directed by Seiji Kishi.

Tropes used in Assassination Classroom (2015 anime) include:

Bully Hunter: Karma beat up a top student for bullying students from Class 3-E. That's how he ended on it.
Cheshire Cat Grin: Koro-sensei default expression is this; he is also a alien that will destroy the world when the times come no matter what.
Death Seeker: At the beginning of Episode 2, Koro-sensei says he hopes his students kill him. But he don't hesitates not even a little in using all resources at his disposal to avoid being killed....
Detonation Moon: And Earth is next if Koro-sensei isn't killed soon enough.
Establishing Character Moment: Returning from suspension, Karma seems to be just a lazy, cheerful delinquent...Until he and Koro-sensei handshake each other and Koro-sensei hand explodes as result of a trap that Karma put in his hand, making him the first student to really hurt their teacher. It shows how Karma is really a violent, smart individual behind an aura of laziness and relaxation.
Friendly Enemy: Koro-sensei and his students respect and even care about each other, but the students will try to kill him or help other people in killing him.
Heroic Sacrifice: In Episode 1, Nagisa decides to take part of a plan to kill Koro-sensei that involves her throwing herself at him with a grenade filled with "Anti-Sensei" material with a powder detonator attached to her. It would severely injure her, but she thinks that the money, respect she would get, and the salvation of Earth would be worthy it.
Kryptonite Factor: The material that can hurt Koro-sensei is perfectly available; the problem is fatally hitting him with it.
Kryptonite Is Everywhere: To be fair, the threat that is Koro-sensei is a closely guarded secret and anyone other than world leaders let in on that secret is more likely than not supposed to kill him and has been supplied special weapons suitable solely for the task of killing him (in fact often these weapons are unsuitable for killing humans).
Nigh Invulnerability: Koro-sensei.
No Endor Holocaust: Considering that 'only' blowing up the Moon to the extent that it was rendered a permanent crescent would be enough to initiate at least a Class 3 Apocalypse in Real Life, the inhabitants of the Assassination Classroom universe sure are lucky to be anime characters. And that Koro-sensei didn't really want to kill any of them, least of all his precious students.
No Name Given: Koro-sensei is initially unnamed until the end of Episode 1, until Kayano names him this way because he's practically unkillable.
Reality Ensues: Not the Fall That Kills You. Koro-sensei saves Karma from a fall not by grabbing him in mid-air in high speed like Karma expected, but by constructing a net made of the same material as his tentacles, saying he couldn't grab Karma exactly because of the changes in acceleration.
Revenge by Proxy: Terasaka, Muramatsu, and Yoshida are threatened by Koro-sensei with this (over his families and friends), because their plan to kill him using Nagisa made him furious, but part of the deal for him to be teacher prevented Koro-sensei of outright hurting them.
Skewed Priorities: Koro-sensei cares about covering the damage he did to a flower garden of his students, despite the fact he wants to pulverize the planet where these flowers grow, making such effort useless.
Super Speed: Koro-sensei is said to move at a speed of Mach 20.