"We are Assassins. Our Target is: The Teacher!" -- Nagisa Shiota
Assassination Classroom takes an idea that shouldn't work, and makes a memorable Manga/Anime out of it: A Dramedy about Middle School Classroom in which the students are being trained to assassinate their teacher!
The story Assassination Classroom starts when the middle school Class-E get a new teacher. An Octopus creature who tells them that he's the monster who destroyed half of the Moon just the other day, and he's going to do the same to the Earth in a year, unless he gets killed first.
The thing is, this Octopus-creature moves at 20 Mach speed and nothing is able to touch him. He makes a deal with the governments around the world to be allowed to teach these specific students how to kill him. They agree with certain conditions.
Thus starts the year in which Class-E have to learn assassination on top of their usual study sessions. Koro-sensei not only teaches the students how to kill, but also how to live... The students soon learn that killing the teacher would require more than the skills needed to kill him, but the resolve to do so as well!
There's no main character in this series, (not in the traditional sense,) the whole classroom shares the main character's role. But there are three characters that get the most focus: Nagisa, who narrates the series and we see most of the events from his view point. Karma, his friend, the ace and the troll of the group, and most importantly: The Teacher!
Koro-sensei is an interesting case because he's both the main protagonist and the main antagonist of the story. While the students are collectively the main character, Koro-sensei is the one that drives the plot forward and he's the connection between all the different plots of the series. He also has most of the heroic traits that he shares with the main characters of other shonen series.
Despite being heroic however, Koro-sensei is the main antagonist of the series, as he keeps reminding the students (and us,) that he's going to destroy the earth if no one kills him in a year.
Also, can you call someone who teaches children how to kill a "good person" ...?
Koro-sensei might seem invincible as no one can touch him, but he has a lot of weaknesses that the students keep discovering through-out the story. For example: The students have weapons made of a substance that could kill Koro-sensei when touched, but can't harm any human if they got stabbed with them. Most of his weaknesses however are his personality flaws and they're played for comedy as much as they're played for drama!
Other character outside of the class and its teachers are interesting too. Since the existence of Koro-sensei is a secret between the government and principle of the school, the only other people who know about him are the assassins hired by the government. It's nice to see how the general public reacts to the effects of what Koro-sensei does. (People can't see him because he moves at 20 Mach speed.)
Many of my favorite characters are antagonists who want to kill Koro-sensei before the students, and it's nice to see them learn his weaknesses and use them to their advantages! It says a lot about how good this series is when I keep cheering Koro-sensei to overcome his weakness and defeat them despite that his death will be good for the world overall.
In my opinion, the greatest strength of Assassination Classroom is that it makes you forget the stakes. With each episode you forget more and more that most likely there won't be a happy ending. With each episode you forget the heavy implications of allowing children to use words like Kill, Murder or use Knives. With each episode you come to care about the killers and the one they want to kill. Then something happens, and you remember all of the stakes at once which makes the emotional scenes more powerful.
The Manga spans 21 volumes, and the anime adapts the story in only 2 seasons. The pace of the the anime feels just right. The contrast between animations of the happy and dark scenes is striking in the best way possible. (The story gets increasingly dark near the end, but it somehow manages to keep the "positivity" of the early episodes/chapters.)
There are many things I could talk about in Assassin Classroom. All the students have their unique personality and each of them receives at least one sub-plot through-out the story. Koro-sensei's character is so unique and complex that it deserves exploration, and I would love to do that sometime in the future!