I think a lot of summaries get too hung up on the 'assassination' part and make it sound like a pure action romp. At its core, 'Ansatsu Kyoushitsu' is about a group of kids society has written off—Class 3-E, the 'End Class'—being given a purpose and a nearly impossible goal: kill their teacher, Koro-sensei, before he destroys the world. The twist is that Koro-sensei is the most dedicated, insightful, and genuinely caring educator they've ever had. So the plot becomes this wild balancing act of learning advanced combat and assassination techniques from your target, while he simultaneously teaches you math, literature, and life lessons to help you become better people and succeed against the elite main school.
The real tension isn't just 'can they kill him,' but 'should they,' and 'what happens to us after?' Each student has a personal arc about overcoming their insecurities or societal baggage, all framed by this absurd, high-stakes classroom dynamic. The government's involvement and the rival assassins that show up add external pressure, but the heart of it stays in that rundown classroom on the mountain.