* AlternativeCharacterInterpretation: Is Nagisa a bloodthirsty sociopath who constantly hides his true nature behind a gentle exterior? Or is he a genuinely kind student who unknowingly has the skills and mindset of a professional assassin? The series seems to lean towards the latter interpretation, but it's hard to be perfectly sure…
* AudienceAlienatingPremise: Even though the series is a light-hearted sci-fi comedy with a CrazyAwesome SaveOurStudents plot, it ''does'' hinge on a classroom full of kids learning how to kill their teacher with guns, knives, and the occasional explosive. [[UsefulNotes/{{Columbine}} Obviously]], it won't fly high in North America and other countries with histories of student rampage. Emphasis on "high": NoExportForYou has been (for now) [[AvertedTrope averted]].
* CompleteMonster:
** Takaoka. Not only was he willing to beat up any unruly students, but [[spoiler:once they oust him he opts to steal the money meant to be used for the assassinations in order to buy a biological weapon and hire a bunch of hitmen. The virus will eventually cause those infected to be covered in horrific tumors and suffocate. He'd also planned to take Kayno and Nagisa hostage. He was planning on burrying Kayano along with Koro-sensei in a bathtub full of cement and anti-Koro sensei materials in order to force him to sacrifice himself to save her, and take his revenge on Nagisa. He also destroys the antidote during his rematch with Nagisa.]] Fortunately, [[spoiler:Gastro, Smog and Grip are {{Punch Clock Villain}}s with [[EvenEvilHasStandards standards]], and Smog never poisoned them with a dangerous poison to begin with]].
** Shiro. He does not to care endangering middle schoolers to kill Koro-sensei. [[spoiler:This white-clad guy abandons Itona after his third loss and leaves him dying, and when he decides to "bring him back", Shiro uses him as a decoy and tries to kill ''both'', him and Koro-sensei.]]
* CriticalResearchFailure: [[http://www.mangahere.com/manga/ansatsu_kyoushitsu/c003/10.html Mach 20, or twice the speed of sound.]] Mach 1 ''is'' the speed of sound.
* EnsembleDarkhorse: Karma is a very popular student among the fandom, besides Nagisa. It helps that he's so badass, and not a complete jerk.
** Karasuma is also very popular, due to being nice and badass.
* FanPreferredCouple: For slash, the most popular couple is Nagisa/Karma. For hetero pairing, we have:
** [[TheHero Nagisa]]/[[TheChick Kayano]]
** [[BullyHunter Karma]]/[[MadScientist Okuda]]
** [[OnlySaneMan Karasuma]]/[[ButtMonkey Bitch-sensei]]
** Sugino/Kanzaki.
** Chiba/Hayami.
** Isogai/Kataoka.
* FunnyAneurysmMoment: One chapter has an image of a man taking a bite out of an Anpanman caricature. Takashi Yanase, the creator of ''Franchise/{{Anpanman}}'', passed away during the same week that this chapter was published in ''Magazine/ShonenJump''.
* GeniusBonus: In Chapter 53, the English 'monster' is covered with the words of the actual text of the question it represents, and the students' attacks leave an imprint in the form of their answers. Not only are the questions and answers written in perfectly grammatical English and Japanese, [[spoiler: they match the explanation given for the reason behind their difference in effectiveness: Seo's answer is written in the sort of formal language you would expect to see on a test, while the question and Rio's answer are in [[TheCatcherInTheRye Holden Caulfield's]] informal, slightly dated narration style.]]
* GodModeSue: ''[[AvertedTrope Averted]]'' with Koro-sensei. Yes, he is an extremely powerful lifeform, with NewPowersAsThePlotDemands but he has a lot of weaknesses, his personality has many flaws, he was even almost killed in the ''third'' chapter, and Karma successfully injures him in the ''fourth''. There are often situations where he is the ButtMonkey of jokes. He helps his students to kill him, but he never makes it easy for them, though they have still come close several times. Whenever there are conflicts, he helps his students, but they are the ones who finally solve them, [[FridgeBrilliance because they are supposed to learn how to solve their problems on their own]]. Koro-sensei is not completely invincible either. His students end up coming to save ''him'' during the Pool arc. And even his [[spoiler:Ultimate Defense Form is a weakness for him because he's immobile in this form]], so he suffers from DeusExitMachina during the Assassination Island arc. And even as a teacher he is not perfect, as the students are not able to triumph in the first midterm exams like he wanted.
* LesYay: Kataoka saving Kayano in the pool. The latter even says that Kataoka was so cool she think she might have fallen for her.
** Kayano makes a similar comment to Yada again.
* MagnificentBastard The school Chairman, who deliberately created a system that would degrade and humiliate a small group of students for the sake of ensuring that everyone else does well. When he realizes that Class E is about to prove themselves, he out-and-out ''cheats'' by making it so that the rest of the school had an overwhelming advantage against Class E. That is, his personally teaching the subject matter of the tests, which was changed at the last minute to ensure the preservation of his damn system. All with a smug smile on his face. He even states that maintaining his system is more important than saving the world. Seriously, '''[[ThisIsYourPremiseOnDrugs THE GUY IS A TOOL ON STEROIDS!]]'''
** Oh, and chapter 23 shows that the system even allows students not in Class E to get away with ''literally'' assaulting and battering a student from Class E. He also has the gall to casually drive up to the victimized student and hand him a handkerchief with a smile on his face, and let the bullies go on their merry way scott-free.
** Then in Chapter 35 when Class E has a chance of winning a baseball game, he comes onto the field and BRAINWASHES the good baseball team to be more aggressive just to keep up his system. Good lord, this system is life or death to him.
** His Magnificent Bastardy is even extending to their training, with him introducing [[SadistTeacher Akira Takaoka]] into the class. And approving a nightmarish schedule of training, which just happens to not leave them any time to study.
*** [[spoiler:Only for the Chairman to later pull Takaoka out of the class, just to remind everyone else who was really in charge at the school. He wasn't about to let the students or Karasuma have the satisfaction of getting rid of Takaoka themselves, after all.]]
* MemeticMutation: Thanks to some memorable one-liners from the students.
** [[BadassBoast Perversion...can save the world.]]
** [[RunningGag He's an]] {{ikemen}}!
* {{Moe}}: After Koro-sensei's upgrades, Ritsu's moe level went through the roof.
* ValuesDissonance: Unlike most of the students, [[spoiler: Yuuma Isogai]] was placed in Class 3-E for having a part-time job, which was regarded as serious a violation of the school regulations. Though this looks like another one of Kunugigaoka's ridiculous, unfair policies to some readers, it's actually not uncommon for schools in Japan to forbid their students from working.
* ViewerGenderConfusion: Some fans are unsure as to whether Nagisa is actually a guy. His slim build and pigtail-like AnimeHair don't help. To make things worse, in the vomic his voice is clearly that of a female, and in the OVA he sounds even ''more'' feminine.
** It finally gets lampshaded in chapter 43, when the class goes for a swim in their school approved swimsuits. A few girls who finally see him topless note that it was really hard to tell until that point.
* TheWoobie: All of Class E can be considered this after seeing the way they are treated like pariahs by students and teachers alike; they are basically punching bags for the rest of the school. Luckily, Koro-sensei is there to help rebuild their confidence.