i don’t get it

Any specific part or the whole story? There's a lot of hints dropped everywhere and various scenes allude to what's going on.

the whole thing pls. i’m very slow when reading and take time to understand the story so i usually read it a few times but this one i still don’t get it.

The principal, Yuuzutsu, maintains their immortality (or prolongs their life) by swapping bodies. It seems to be implied that the "teacher's pet" position is the next body they will take. This is why the MC questions why Tatsumi wants to be it so badly. This is because Tatsumi is naïve and only wants the spot out of admiration, not because he actually wants to give up his body for Yuuzutsu. This admiration slowly evolves into fear because he starts learning after watching the MC. The whole concept of Yuuzutsu maintaining immortality/life is hinted at in two major scenes and constant subtle hints all of which is confirmed at the end.

Major scene 1: The scene with the fish experiment. A fish head from one is attached onto the body of a second fish. This is where the MC questions Tatsumi and asks him what he thinks about the newly created entity. Is it just one fish? Where does the soul go? What happens if it only one fish soul survived? Where did the second soul go?

Major scene 2: Tatsumi's memory from when he was young with a bird that had a broken wing. This is where they explain why Tatsumi admires Yuuzutsu because he "fixed" the bird. It's not explicitly shown that Yuuzutsu attached a different bird wing, but the before and after scenes show a perfectly healthy black bird in the cage then after the bird Tatsumi saves flies away with a new black wing, the cage is empty with black feathers on the floor of the cage.

Subtle hints:
1. Yuuzutsu never shows their face or hands and if you do look at the panels where they appear, the way the character is drawn is like a decrepit old man.
2. MC brought in the first question to his friend if you're offered bread while starving but you're asked to exchange your life, would you take it? This was just simply foreshadowing and the first piece to help you literally piece together what the MC's background may be like.
3. MC always asked for the story of the witch to be read, yet ends up with a nightmare often. I can't remember the specific reason why this was enforced on a character level, but I think it was more for us readers since it's forced foreshadowing at least in my opinion.
4. The medicine MC took seems to muddle with his conscious to the point where they seem aloof and dead like an empty husk.

And probably other subtle hints I didn't list here that I can't recall.

Everything is confirmed and explicitly stated to readers in chapter 5 where the MC botches the surgery and effectively either kills Yuuzutsu or puts Yuuzutsu into a permanent coma. MC did it stating that if he didn't do it, Tatsumi might be next (disregarding his own survival), but Tatsumi also pointed out that he thought MC died. This meant that the conditions for swapping bodies is that the new host needs to be more or less an empty husk and Yuuzutsu transfers themself onto the new body by transplanting their head(?) just like in the fish experiment scene. This ties everything up.

OHHHH man i would’ve never gotten the plot if u didn’t explain it. THANK YOU SO MUCH!

you’re a life saver thank you