I thought the comments were crazy, but after chapter 51, I think we can pretty conclusivel...

I thought the comments were crazy, but after chapter 51, I think we can pretty conclusively say that the story has been now split into two timelines:

So-Myeong timeline and Som/Mother timeline

I kept trying to look for slightest physical differences like height, eyes, or uniforms, but there aren't such things. So I'll present the actual differences here:

Characters in So-Myeong timeline:
-So-myeong, Hyojin (her black haired friend), Siyun, red head fuckboy

Characters in Som/mother timeline
- Som, Sol, Killing-stalking-wannabe barista

Changes in So-Myeong and Som:

-So-Myeong never cared about smoking, but Som seems to hate it and judge people on it all the time
-Som is in general more "upper class" with her taste, she won't sit on old chairs and is grosses out by greasy food
-So-Myeong pushes her friends away to protect them, Som is extremely possessive
-Som complains about her daily life being tedious and repetitive, So-Myeong is tired of being on edge all the time. I think So-Myeong would enjoy being a boring adult very much
-Som is very chill about her mom "My mom would understand if my bag got wet in this weather" "It's raining, should I call my mom?" "I put the umbrella in the hall, but my mom must've put it in my room""I can't go out, my mom made dinner, haha okay you're funny I'll have dinner with you", So-Myeong is scared of getting murdered
-The way Som meets her lunch mates is almost identical to the way So-Myeong met her now murdered friend, so I don't think she'd be eager to repeat that
-Som lies, she lies willingly and often. We can compare the situation where a bully was being falsely accused. Som cried and said she doesn't remember, So-Myeong stayed silent. So-Myeong lies also, but mainly about her mother or to her mother.
-In what universe would So-Myeong keep an unlocked box with all things important to her in her room, and out a photo of a "low quality" friend in there, when she knows her mother would immediately find it?
-So-Myeong wants to avoid confrontation, to the point of concern of her friends. Som just met this girl and ended her bloodline, and taunted her on top of it.
-"I hope Sol and I create more memories during these fun times!" So-Myeong isn't having fun times I think haha
-The chapter 51 line pretty much confirms it "I want to do something about my hairline, get rid of my bangs and cut my hair into a bob" So-Myeong DOES NOT want to look like her mom lol
-So-Myeong's brother and Son's brother are not the same characters, their hair is drawn differently. I'd understand So-Myeong maybe wanting to lie about her dead brother now, but the flashback with her studying with him never happened. Her brother was a little dumb and she loved him for it.
-I don't think So-Myeong would trust the police to handle parental abuse of her friend
-Som is just a lot warmer than So-Myeong has the energy to pretend to be.
-Som doesn't have So-Myeong's disorder I think?So-Myeong looses her sense of consequences completely, Som just gets meaner, but she still thinks things through. Baiting a girl you don't like to say something stupid and then tricking the teacher into letting you off isn't that and it doesn't serve So-Myeong's plans

Smaller changes:
- So-Myeong's mother says she caught who was smoking outside their house and So-Myeong isn't concerned at all. Why would she? It's some random college student and not Sol, who she doesn't even know.
- At So-Myeong's school, nearly every main character has some gossip floating around "he's a real piece of shit!" "He's so handsome!" "She's so kind!" I think there'd be some rumors about Sol, a girl who smokes and struggles with studying and has an older boyfriend on top of that.

If you've read this far, thank you, let me know if I missed anything and your theories on what So-Myeong found in the closet.