This is a fun book. Here's something I wrote about it:
This is a really cute book about a group of students who try to kill their teacher everyday lol. But they have to because the teacher is some kinda alien who vows to destroy the whole planet if he's not killed. So different students try different methods--shooting him, stabbing him, poisoning him--but nothing works because he's really smart and fast. He's shaped like an octopus and is also an actually good teacher. He teaches clearly and gives these kids a lot of confidence. He notices them. And that's relevant to the class too because it's a group of kids who don't get good grades and got sent to this other building. So it matters that Koro Sensei actually teaches them, despite the fact that they want to kill him.