I enjoy this manga still but I do have to agree the children in this are not talking or behaving like children or teens in this case I would believe if there were some kids in her class that are very mature for their age but just not everyone she talks to. They sometimes talk like they've been reflecting on their lives for decades.
[...]
A lot of this content type has that issue. A lot of times when I read manga or watch anime with kids of various ages, I am flabbergasted with what type of topics and conversations they have. Like I don't remember having a single such serious/heavy topic being conversed by me or my friends, even at university stage. You can barely hear something like that talked by like 30/40/50+ adults - people in general have "better things to do" than to go full psychological/etc. At worst its just talking about political issues and how stupid politics fuck their day-to-day lives with rising taxes, expensive insurances, cars breaking up and being expensive to fix, etc.
Heck even at end of High School we spent more time just having fun for example meeting up and doing like low people count lan parties, where some of my friends would even out right bring their PCs including a fucking CRT monitors. And on the longer ones my parents would bring us like a 2L box of ice-cream and just a bunch of spoons to eat it, etc. No existential dread talks there, just what CS map to boot up next, or what mode/map to play in SC.
Or maybe I an my friends were simply weird?