Why is most yuri set in all-girls schools?
Note: this is a work in progress, and it’s possible there are some minor inconsistencies or errors. I’ll progressively come back and revise it. Also sorry, I skipped over the 50s-60s for now. Sakura Namiki (The Rows of Cherry Trees) is an important work from then.
Yuri’s distant ancestor is Class S (a literary genre and culture featuring romantic friendships between girls). However In a modern context, Shiroi Heya no Futari (which carried some influence from Class S & earlier shoujo novels) is really what solidified that setting, and many common tropes. Here’s an article on the 70s onwards portion of it: Erica Friedman 70 Years of the Same Damn Story
Historically…
Girls secondary schools (early 20th Century) were a new thing (initially only affordable to the wealthy)… And the whole 少女 shoujo culture was created from these schools and girls magazines where the readership was almost (if not entirely) all girls. It was basically a temporary bubble from stuff like marriage and employment. The schools were a safe place where girls could develop very intimate, quasi-romantic relationships (that at least in theory didn’t cross the line into sexual stuff… but let’s be real).
The girls schools came out of legislative reforms that were intended to help Japan modernize (to Western influence and morals), and this worked in conjunction with the whole 良妻賢母 (good wife, wise mother) policy/outlook. Consequently the magazines were monitored by the government, so overtly homosexual content wouldn’t fly (especially in later years). And part of what emerged from that is a pure love that was kind of “for practice”, and technically ended after graduation - though in practice, many women in the stories and real life maintained those relationships in some form throughout their lives.
This is also the culture that Takarazuka Revue emerges from - because it was a “safe” theater, by virtue of its all girls cast. One of the defining aspects of Takarazuka Revue is the 男役 otokoyaku - where male characters were played by androgenous women - who were a sort of idealized form of masculinity, more (ideally) masculine than actual men, while still being female - and thus neutral & safe (Oscar from ベルサイユのばら, Haruka from Sailor Moon, Utena, etc all derive from this. And even in ベルサイユのばら, one of the significant male characters essentially has to be de-masculinized before his love for another character could be fully accepted).
Anyway, that all-girls space and elite private school setting still carried over years later into 70s shoujo yuri (ex 白い部屋のふたり Shiroi Heya no Futari (which really solidified that setting and various tropes) or おにいさまへ Dear Brother (which deconstructs that)). Even when the readership for yuri started to diversify, and 男性向け male-demographic yuri started getting produced in the 90s, it largely kept that default setting (despite the mixed gender settings of shoujo series like Sailor Moon & Utena paving the way for late 90s/early 2000s yuri).
While a lot of modern yuri go so far as never even having a male character shown (404: Men not found), the pre-war Class S and 少女小説 shoujo novels (pre and post-war) and 70s yuri definitely includes them as side characters. I’m not 100% sure when that changed… probably in the 90s, due to largely emerging out of 同人誌 culture. I think the stories in early Yurihime editions (back when it was a shoujo magazine) generally maintain this - but I’d have to double-check, and certainly Yurihime S (the now defunct male-demographic version) did.
Side note: despite the 良妻賢母 Good Wife, Wise Mother origins of shoujo culture and magazines, they took on a life of their own, and created actual physical and virtual communities that fostered a lot of personal growth, wisdom and professional development for women at the time - and helped give them more independence than they’d previously had. Even here, the editors were all men, but (at least in the case of 少女の友 magazine) when they answered letters and such, they wrote in the same style that the girls wrote to them in - again the whole de-masculinization thing, like with Takarazuka Revue. (In the more conservative 少女画報 that was not the case… But that magazine was more aimed at parents anyway)
The info above is an amalgation of things I learned while reading Passionate Friendship: The Aesthetics of Girl’s Culture in Japan by Deborah Shamoon, Age of Shōjo: The Emergence, Evolution, and Power of Japanese Girls’ Magazine by Hiromi Tsuchiya Dollase, and By Your Side: The First 100 Years of Yuri Anime and Manga by Erica Friedman. Any errors are my own