In this particular instance, Lelouch needed to hide his identity, and being able to go places as a regular Britannian citizen helped him scope out defenses or perform other covert work. a big theme of code geass was also how the youth of these children was stolen by imperialism, both the Japanese but also the colonizers. But more generally, what are the characters gonna do if they drop out of high school. Josuke from Jojo doesn’t even have enough money for nice shoes, let alone trying to fund himself on stand user adventures. Kakegurui doesn’t make sense in real life but it isn’t supposed to, it’s all the insanity of crazy gambling mixed with the feelings of dread most Japanese high school students have. Blue Exorcist, my hero, and Soul Eater are about schools set up to teach the super powered youth how to use their abilities. Bleach and death note both have a world apart from the real world, so the characters keep up their civilian lives. Percy Jackson, Harry Potter, enders game, and the mysterious benedict society all reflect one of these to an extent. Honestly most western comics are pretty similar as well

I think it’s mostly about not having to spend the budget on backgrounds because every Japanese high school classroom looks the same

Also it’s probably really easy to steal a high school background from one of the countless other anime set in a high school and get away with it, just trace over it and change a few things