>>7837School provides an easy contained setting that is relatable to pretty much all readers: children are living through it, and adults can remember their days going through it. It follows a strict framework that makes it easier to write, given the expected conventions of tests, clubs, school trips, etc., and is thus very convenient for many genres (romantic comedies being a prime example, but not only).
As anons above have pointed out, the Japanese in particular have a weird idealized version of school in their culture whereas it takes this sort of landmark place as "the last time you could be free" since labor in Japan is so nightmarish that once you enter the working force your life becomes hell. This, of course, isn't entirely correct; school life is often quite terrible, with ungodly pressures from absurd and constant testing, tons of drama, bullying, and misery all around.
As I see it, school is chosen as a setting so constantly not for its concrete characteristics, which very few works actually have the guts to delve into ("delinquent" manga often being a prime example), but to follow an easy framework that feels like a comforting narrative device, guaranteeing the reader that nothing really unexpected is bound to happen, and they can read with certainty that their beliefs probably won't be questioned. It's form convenient for a work aimed at mass cultural output. This, of course, becomes absurd in those manga that have schools with like ten thousand students that are functionally a self-governing society, at which point the plot device unwittingly becomes a parody of itself.
The easiest way to demonstrate the way school is merely a convenient plot device is by the problem of parenting and reproduction (in the marxist sense): how many manga set in school have to come up with contrived plot devices (working overseas, not giving a shit about their children, just plain never appearing) in order to remove parents from the narrative, and thus not have to grapple with the actual reality that being school-aged entails: that the "freedom" of schoolkids is in real life anything but, given they must answer to several different authorities they have no meaningful way to recourse against.
I find that for better understanding of these kinds of plot devices reading "How to read Donald Duck", a marxist classic by some folks in the Allend
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