I'm not too clear on what this is supposed to be either, but there are a few details that narrowed the possibilities down a bit. The horrific details seem to only appear to the boy, hence the classmates still laughing and the teacher being unfazed. When the Somegi appeared, there was an emphasis on stabbing and slicing, like someone being murdered with a knife, along with the phrases "don't run away" and "don't look away" being in the same text box as what seemed to be the victim pleading for their life. The subject matter of the story also seems somewhat interesting, with it being overly emphasizing tragedies befalling a monkey (the closest animal to a human) because said monkey was "mean". The teacher's seriousness when Takeru talked to her was also a bit interesting to me, especially after she was shown to be completely careless to the contents of the story. With those details in mind, I thought about a few possibilities.
1. The boy has trauma due to witnessing a horrific event, what I presume to be a murder by knife carried out by his mother against someone else, possibly his father but there's nothing solid on that front. I say father though, because of the authoritative tone of the monkey when it goes from pleading for its life to commanding the reader directly, probably just Takeru due to him being the only one reacting, it comes off more like a parent, and his mind came up with the phrases "don't run away" and "don't look away" to justify him being unable to do anything at the time. The reason the teacher is relieved and says it'll be alright is because Takeru can't remember the specifics, likely due to details being locked away in his brain to protect him from the truth, meaning that because he can't remember it being his mother, that means he won't have to experience the resurgence of trauma when the metaphorical dam breaks in his mind and he recalls the events in full. However, this also leaves him with a punishment complex, especially if his parents are now nowhere to be seen and he has no clue why, coming wrongly to the conclusion that it was because of him being "mean" to them, hence why the story had begun to shake him.
2. The monster details did call out to me even though I leaned more towards the trauma route, so it could also be that a creature known as a Somegi had begun taking control of his mother at home, and the possession was showing itself through her skin being slowly coated in a viscous black substance not unlike ink. It's possible that the murder did still occur, but it could be that the creature had separated from the mother, possibly to kill her and finish taking on her form through absorption of her corpse, something it would be incapable of while she was still alive and capable of resistance. It's even possible that Takeru was the one who killed, that he had taken on the Somegi before it could fully take on his mother's form, and afterwards, he was brought to a school to recover and get used to normal life again, with the teacher being familiar with the monster perhaps due to cases like his in the past.
3. Going away from the trauma angle, I also considered it from the classical horror perspective. Instead of his mother being possessed, it's possible that Takeru himself is under the possession of the Somegi, with it not being a physical monster, but a monster contained within his mind, corrupting his world and slowly driving him away from sanity. The reason the text boxes didn't change despite the 'monkey' going from a character's voice to one directed at Takeru is possibly because it was the Somegi using the teachers voice to tell the altered story, and when Takeru began to look away in discomfort, the Somegi chastised him, trying to force the boy to look at the horrors it had concocted possibly to give itself a way to take over the boy's mind. The teacher's words at the end could also be the Somegi gaslighting Takeru into thinking there was nothing wrong while further scaring him with details of a monster, but it could also be the case that it was actually her talking to him, and because the Somegi was formless, she somehow knew that it was a weaker monster incapable of manipulation through taking on other peoples forms.
There aren't really enough details to come to a proper conclusion, but these three general ideas made the most sense to me.