Assassination Classroom

After destroying the moon, an inhuman nigh unkillable creature has decided to destroy the earth in less than a year. For some reason, he has decided to become a High School teacher and under no circumstances will harm his students. His students now have to kill their teacher before he destroys the planet. This may be more difficult than expected, however, as not only Koro-sensei (the name they decided to call the creature) night unkillable, he also is the best teacher they have ever had.

Assassination Classroom is a manga series written and illustrated by Yūsei Matsui. It originally ran in Shonen Jump from July 2012 to March 2016, compiled in 21 volumes, and is licensed in North America by Viz Media. It has had two anime adaptations, a single OVA episode in 2013, followed by a TV series in 2015. It also has two live-action adaptations, released in 2015 and 2016.

Tropes used in Assassination Classroom include:

Ascended Fanboy: Inverted. The "God of Death" was inspired by another hitman to become a hitman. Which hitman? The one who killed his father.
Asshole Victim: How the "God of Death" describes his father. Considering he is a jerkass in the present even by hitman standards, it sort of falls in Unreliable Narrator territory.
Berserk Button: Koro-sensei goes to Brazil during the 2014 FIFA World Cup, and accidentally mentions the numbers 7 and 1 several times. The thing is, Brazil just lost the semifinal match of the cup in their home by a humiliating score of Brazil 1 x 7 Germany. The result is lot of pissed off brazilians.
Cloudcuckoolander: Koro-sensei says some odd things sometimes. Like saying that dressing as a dog makes him more able to feel scents.
Damsel in Distress: Bitch-sensei is kidnapped by the hitman Shinigami, who forces the heroes to try to rescue, since otherwise the Shinigami will cut her into pieces.
Detonation Moon: And Earth is next if Koro-sensei isn't killed soon enough.
Dissimile: In Chapter 104, Yukiko says Bitch-sensei don't would be the kind of person to go through a Face Heel Turn. She begins to describe her, but then realizes Irina is the Token Evil Teammate and agrees she's exactly the kind of person that would betray the class.
I Fight for the Strongest Side: Inverted. Bitch-sensei goes through a Face Heel Turn because she believes the "God of Death" has more chances of killing Koro-sensei than the class.
Improbable Aiming Skills: The "God of Death" can hit someone's aorta with a very small bullet.
Innocently Insensitive: Koro-sensei accidently starts a riot against himself by mentioning 7 and 1 out loud a lot of times to grieving brazilian supporters. They try to beat him up.
Groin Attack: To put it in simple terms, Karasuma kicks the "Shinigami" in the groin.
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).
Mini-Dress of Power: At first seems inverted, since Bitch-sensei is wearing a mini-dress after being kidnapped by the "God of Death". Then she revealed she was allied to him all the time, and proceeds to fight her students.
Names to Run Away From Really Fast: A hitman called "God of Death/Shinigami", period.
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.
Nosebleed: Irina gets one when see Karasuma without a shirt.
Playing Possum: Downplayed. In Chapter 104, Irina pretends to have tripped and injured herself to attack a group of students that still think of her as her weird, kind of a jerk, but still goofy teacher.
Stepford Smiler: The Shinigami talks in a friendly, calm tone towards the class. Hinano even thinks he may be a good guy. Karma, however, says that is just a way of seducing his targets before he goes for the kill.
The Needs of the Many: The "God of Death" creates an assasination plan for Koro-sensei that would end with the students also killed. Karasuma is caught between his responsibility towards Earth and his love to the kids but, ultimely, decides they're worthy the risk the possible future destruction of Earth.
Unwitting Pawn: The "God of Death" is said to use only this kind of ally to execute his assassinations.
Would Hit a Girl: The "God of Death" would kill it even.