Some of Koro-Sensei's reactions to being attacked, for instance when Karasuma tries to strike Koro-Sensei with a sword he gets his eyebrows trimmed, meticulously even.
When Irina tries to tell the class to call her Jelavic-Oneesama instead of Irina-sensei, the class decide to instead call her Bitch-neesan (Due to Vic and Bitch sounding similar when spoken with Japanese phonology). Irina is not amused. She is certainly even less amused when they proceed to call her Bitch-sensei after she finally manages to win over their approval.
The first English line Irina teached to her students was "You're incredible in bed".
Koro-Sensei and his interactions with the students tend to be this or CMOH. Particular case is when he lets them tie him up in a tree so they can try and spear him with make-shift spears (after he had uprooted the tulips they had so dilligently raised) and then he panics after falling onto the ground where they could actually get him.
Koro-Sensei in general is also pretty funny.
So as Ritsu.
Chapter 11 is full of these, but particular mention is those two guys' reaction to seeing Nagisa receive a Marshmellow Hell from Irina.
Tadaomi quickly tears them apart and pins Irina's arm behind her back, leading her away from Nagisa.
And in Chapter 8, there's the 30-hit combo french kiss.
In chapter 11, there's also the scene where Koro-sensei arrives on the stage. Irina casually stands next to him and tries to stab him repeatedly, but Koro-sensei dodges each attack. Then Tadaomi once again pins her arm behind her back and leads her away before she blows their cover.
There's also a moment where some of the girls from Class-E come up to Tadaomi and show him their bedazzled knife sheaths, asking him if they look cute. His response is to tell them to never take them out in public, even if they are cute.
Koro-Sensei's "Bubble bath" in chapter 19. Especially how he ends it. Some students are blocking the entrance, trying to get a look at his naked body. ("I can't believe such unsexy peeping exists in this world.") To escape, he just stands up, but the water has become jelly-like because of his mucus so it lifts up with him in a solid cube. Then he jumps out the window, cube and all.
"You made a 2-D girl cry".
Irina's incredible awkward act to get near to Karasuma.
"Even a cabaret girl would act awkwardly if she suddenly had her father as her customer, y'know?"
Karasuma proved himself to be not so weak after all and scared the shit out of Irina and Koro-sensei.
The similarities between Koro-sensei and Itona.
Chapter 35 reveals the one thing Kayano really hates: big breasts.
While Karusama is profiling the school chairman, Irina gets a "eureka moment" when she figures out what "outs" in baseball are.
In chapter 37, Sugaya comes back with a tattoo on his arm. It turns out to be a traditional Indian Henna tattoo, which is temporary. We quickly cut to Koro-Sensei, who was in the middle of reading a mountain of books on what to do when one's child becomes a delinquent.
The hilarious reaction of the screaming Koro-sensei, whose face is melting, and the screaming students when Sugaya tries to assassinate him with the henna paint on Koro-sensei's face. The faces of the studends are drawn in a completely different art style.
Most of Koro-sensei's weaknesses.
He loves boobs.
Hackneyed plot developments in movies make him cry.
He's very petty.
He doesn't like hot food.
He's easily taken in by rumors.
When he returns to normal after a serious fight, he gets really embarrassed.
He's very worried about appearances.
He can't swim.
At one point Koro-sensei teaches all the students in class by moving fast enough to create tangible afterimages and using those afterimages to tutor each student. One student is worried that it would exhaust him. Koro-sensei tells the student to not worry, because one of his afterimageswas in charge of resting.
In Chapter 42 when Irina asks the class what she is to them, Takebayashi simply replies, "Our bitch".
At the end, Karasuma decides to treat the class to whatever food they want. Korosensei wants in but he's not allowed as he didn't actually do anything. Leading to him following the class while bowing dogeza style.
Terasaka dunking Karma in the pool. Karasuma has many Not So Stoic moments, but this is the first time we've seen Karma have one. Afterwards, the rest of the class dogpiles him in the water, annoyed by his Magnificent Bastard attitude.
Korosensei demonstrating the effects losing a tentacle has on his speed. It starts with a few of his copies becoming kiddie versions, and eventually devolved into a family drama about a single mother.
As a result of Class E's good results during the final exam, Korosensei has to let them shoot off three tentacles as his part of the deal. He thinks to himself that it's fine, only for Terasaka and co to march up to him with four more exam scripts showing they all placed first in home economics. Upon realizing he has to let his students shoot seven tentacles, he starts panicking like crazy!
Koro-Sensei having an utter shit-fit when his plan to scare the kids winds up scaring him, beginning with Kirara Hazama's Nightmare Face. Especially when he runs into an "Eyeless monster". Better known as Ryuunosuke Chiba.
Chiba: I have eyes...
Chapter 75: Karasuma-sensei's first weakness: Super dense.
Itona: Hey, Muramatsu. I don't have any money. I'll hold in my puke, so let me eat some ramen.
Chapter 88: Itona builds a mini-tank with a camera to kill Koro-sensei. The boys realize that they can look under skirts of the girls with that tank instead, and the boys create an operation how to develop the perfect tank for that matter. All of them have act with serious faces and have specific roles.
The tank is destroyed by a weasel.
Chapter 89: All of the Code Names are ridiculously funny and fitting.
People in the hospital reacted to the name JUSTICE!! Kimura.