Let's play a game! Imagine you wanted to create an anime that couldn't possibly be shown in the USA! To make the challenge more interesting, let's also say that it has to be broadcastable on Japanese television, it's not hentai and you can't use incest, paedophilia, rape, coprophilia, hardcore tentacle pornography, etc. In outline, this show sounds like the ultimate answer to that question. Imagine a light-hearted action comedy about a class of fourteen-year-olds who try to kill their teacher every day with knives, guns and explosives. They get trained in assassination. They do military workouts and learn about murder techniques. Ten billion yen will go to the successful killer! Hurrah!
Surprisingly, though, the show hasn't been banned worldwide. It's awesome. It's been a hit everywhere. The original manga got both an anime TV series and a live-action movie in 2015, with both getting sequels in 2016.
What makes it work is Koro-sensei. Class 3-E's teacher is a yellow octopoid that can regenerate any damage, fly at Mach 20 and destroy planets. He demonstrates this by vapourising 70% of the moon, before promising to repeat the trick with the Earth in less than a year. March, to be precise. Until that date, though, he wants to be the teacher for Kunugigaoka Junior High School's Class 3-E. So what if the entire human race wants him dead? Let them try. It'll give him a laugh. Koro-sensei (a pun on "unkillable") is such a happy, cheerful soul that he'll polish your aircraft wings while you try to shoot him down with air-to-air missiles. He'll encourage his students in everything they do, including murder attempts on his own life. He'll give them tips! He'll praise particularly imaginative assaults and tell less gifted students where they went wrong.
It's unclear why he's so trying so hard to improve his students when he claims to be going to kill them all in March, but that's Koro-sensei for you. He's a goofball. He's comic relief. He's got an outrageous range of superpowers, but he generally uses them to be a better teacher and instead solves problems with trickery, low cunning and a genius for driving his enemies up the wall in order to make them better people. His students love him (and think he's crazy)... but that doesn't mean they're not sincerely trying to murder him at all times. Koro-sensei finds this funny and admires their pluck.
Mind you, after a while I must admit I did think the students were taking the situation too lightly. The Earth's destruction should trump all other considerations, surely? Defending Koro-sensei against rival assassins should be an insane idea, even for children who appear to have warped "I want to kill him myself" into a declaration of surreal mutual affection.
Maybe they don't think he'll go through with it? After all, we never see Koro-sensei do anything bad (although he's frequently silly). He's a nice octopus. You'd love to have him as your friend and it's quite likely that he's the best teacher on the planet. Besides, he's not invulnerable. Mach 20 is fast, but it's not light-speed. If he flies off to Shanghai to go shopping, the students can calculate how long he'll take to return. He also has weaknesses and it wouldn't be impossible for a sufficiently well-planned murder attempt to succeed. There might also be other beings out there powerful enough to threaten him.
He even looks wacky. He's got a head like a Smiley Face badge and you have to read his emotions from his colour. Green stripes = smugly mocking you. Dark purple with an X = wrong answer. Orange with a circle = right answer. Black = congratulations, you've got him angry.
That's the heart of this show, but there's more to it than that. Kunugigaoka Junior High School uses Class 3-E as the object of institutionalised bullying to motivate the others. If you get sent there, you're a failure. Your life is over. The headmaster fixes the exams to ensure that 3-E comes last, scolds Koro-sensei for letting a 3-E student talk back to a superior student (i.e. any of them) and watches approving as they get beaten up. Koro-sensei vs. this system is a lot of fun, although it will also become clear quickly that trying to push around a class of trained killers isn't a great idea.
(Don't worry, though. There's no actual killing in these episodes, although not for want of trying from certain parties. The show's full of soldiers and hit men with an astronomical body count, but Koro-sensei disapproves of the taking of human life.)
It's less student-focused than most teacher-protagonist shows. There's just as much stress on what it means to be a good teacher, with Koro-sensei acquiring colleagues who in some cases need some pushing to do the job they claim to be there to do. As for the students, they're surprisingly free of emotional issues, troubled backgrounds, evil parents and so on. Koro-sensei isn't transforming their lives, but genuinely just teaching. All things considered, I'd call them pretty well balanced. Only two students really stand out. (Well, three if you count Ritsu, but she's not human.)
(a) Nagisa is a boy who looks like a girl and is thoroughly nice, but underneath that he's also the class's most natural killer. It takes a while to unfold because he's not aggressive at all. He's certainly not a bad person. He just seems to have been born without the usual human reactions to the idea of murder.
(b) Then there's Karma, a red-headed bastard with a ton of intelligence and a twisted personality. He's Nagisa's friend, but unlike him is an out-and-out sadist who enjoys hurting people he doesn't like. (Fortunately his targets tend to be even bigger scum than him.) Karma is lots of fun, especially when it turns out that the headmaster's son is a lot like him. They're fated to go head-to-head, obviously.
What about the morality of all this? Answer: it's completely fine. The whole thing's so fantastical that I think you'd have to be actually looking to take offence to have a problem with it. The children's weapons are all made out of Anti-Sensei Material, which is like foam rubber to humans but the only thing known to harm Koro-sensei. (That's the reason why they're not zapping him with lasers, obviously. Mach 20 isn't fast enough to let Koro-sensei dodge a light beam, but he wouldn't need to.) Also the show's clear on the wrongness of killing actual people, although that said I had slight reservations about the respect shown to those three hitmen in ep.22.
In short, it's fantastic. It's laugh-out-loud funny and irresistibly feelgood in its gleeful anti-taste. It's hardly the most serious show in the world, but Koro-sensei really is helping everyone around him become better people. I also loved the way that the show has indeed been built intelligently around teaching. (Well, when it's not built around assassination attempts and comedy.) Koro-sensei is very perceptive indeed and makes some acute points. Speaking as an ex-teacher, it rang true to me. Give me more!