@Tenc pretty much everything? Its a korean story about korean kids. The attitudes, cliques, goofy friend-violence... Its stuff almost any kid in a modern urban setting can relate too, but its all got that korean feel to it.
Coming from my american persoective, a few key points: sweet potatoes arent a snack over here, but a vegetable you see alongside meat at dinner, usually only during holidays. Most families just do regular potatoes.
Boys hanging out doesnt usually involve one of them being half naked unless theyre really god friends accustomed to giving no fucks about each other's privacy, and they definitely dont share boxers or get that close without making gay jokes to cover up the awkwardness. Girls are more likely to borrow clothes, but thats usually only in elementary/middle school.
Literally everything about heirarchy based on age and seniority would be completely out of place in america, as would any use of honorifics or getting mad at someone for using your given name. If somebody uses your last name at all, its them being extremely formal, or being a complete stranger at first meeting.
The fashion choices are a big one. Contemporary korean fashion pretty much ripped everything from american pop culture/westernization, and thats an interesting topic to look into. However, fashion in america has grown exceedingly stagnant and generic, while korea has kept innovating on things.
Here, a guy has three choices: pants an tshirt, pants and buttoned shirt, or a formal suit. And everything is high waisted skinny jeans or slacks. Or shorts. Pretty much everywhere else, not just korea, theres more variety to things. Girks of course have a lot more options, but again, the variety they have is more than over here.
The general casual look of outfits - oversized hoodies, baggy pants, etc, is something our media has pretty much driven to extinction, to the point it makes you look like a wannabe gangster or hobo if you dress that way. But here youve got kids dressing comfortably and no apparent bias against that.
The funny costumes and school fairs? Not strictly korean, but you only see mascot outfits at sports events, and school fairs just arent a thing at all over here. School security is different too.
In manga you always see schools with walls, fences, and typically a teacher posted as guard. That sort of thing is something most of the usa neber bothered with until recent years, and we skipped right to putting electronic locks on the doors and putting rent-a-cops in the halls, outlawing backpacks, and having cops bring the dogs around to sniff out students' cars and lockers.
So... Lots and lots of little details. Life for a teenager going to high school is basically the same everywhere unless youre in a third world country or an elitist private academy of some kind, but the ambience and attitudes and social aspects are all wildly different.