What is it about those schoolgirls? They have such a strong emotional bond with each other that you'd almost think they're dating. Oh... they are?
Perhaps it's driven by wish fulfillment since girl-on-girl is hot, but there are a lot of stories about schoolgirls developing feelings for each other. The fact that girls also often form romantic friendships certainly helps fuel the trope. In any case, there are a lot of lesbian schoolgirls running around in fiction land. If an All Girl School setting is involved, the girls in the story may actually identify as straight, as many girls at old-fashioned all-girls' schools would enter adolescence without any male outlets for their budding sexuality.
Yuri is the Japanese word for "lily", and also refers to a sapphic romance genre in Japanese media. It can focus either on the sexual or emotional aspects of the relationship. Occasionally, some fans will use the term shoujo-ai ("girl love") to distinguish the less explicit works that focus on romance and/or emotions between the characters. Note however that in Japan, shoujo-ai refers to liking little girls.
Yuri is a genre (or a collection of overlapping genres) distinguished by the use of a set of common tropes and stereotypes. The term "yuri" is sufficiently strongly associated with those tropes and stereotypes that some Japanese lesbian manga fans use a different term, like "bian" (short for lesbian), "Onna x Onna" ("women x women"), or "Onna-doushi" ("women together") for works which are actually by/for lesbian women. Yuri is often an idealized and unrealistic portrayal of lesbian relationships, and is not representative of actual LGBTA culture and people due to its Fluff content.
There are a few different types of yuri, depending in part on the target audience:
- Yuri for men, "seinen yuri", tends to emphasize sweetness, femininity, girlishness, innocence, purity, and Moe generally. It usually has a very romanticized view of what it's like to be a teenage girl. This is in contrast to the standard Girl-on-Girl Is Hot scenarios (lustful women making out with each other for the viewer's delectation, often mixed with heterosexual content in fanservice-driven shows) which are usually not considered yuri. Men's yuri typically fetishizes femininity and youthfulness and rarely focuses on long-standing relationships or adult characters, much less LGBTQ culture and social issues.
- Yuri for girls, "shoujo yuri", may also be hyperfeminine and "pure" but can feature some degree of genderbending or crossdressing, with the bifauxnen in a more traditionally male role. In either case, it's usually focused on fantasies of female solidarity, idealized femininity and gender transgression, not with lesbian identity, culture or actual life in Japan.
- Yuri for adult women, "josei yuri", is usually less stereotyped and fairly realistic, like most fiction aimed at adults. Yuri for lesbians is the most likely to be realistic, and to deal with LGBTQ culture and social issues, but is a very small portion of all yuri produced. As previously noted, some readers use a separate term for "yuri by/for lesbians" to distinguish it from the more common types.
In the past, a disproportionate amount of yuri stories have ended tragically or inconclusively, but this is changing as more fictional couples are allowed to have happy, committed, and canon endings.
In yuri, you'll most likely find relationships that play on Tomboy and Girly Girl or Sempai/Kouhai dynamics. The first usually emulates a heterosexual couple, while the latter emphasises the difference in maturity between the girls. Teacher/Student Romance is also found, but less commonly than the other two.
Before girls start focusing so much on boys, there is a time when friendship is emphasized, usually junior high/middle school. These friendships can have very strong emotional bonds, and when two girls develop a close bond it will almost appear to be a romance. They hold hands. Getting together to do something is a "date." Fights are treated as a "break-up." They are not actually in a romantic relationship, nor will they ever be, it's just emphasized in such a way that if one were a boy, you'd assume this to be the case.
In the Western world this used to be far more widespread up until the late 1800s, and applied to both men and women, even into adulthood. It was referred to as "romantic friendship". This practice waned at the turn of the 20th century, as adults, particularly men, did not want to be mistaken as homosexual. Generally, it only still occurs between young girls, where it is more socially acceptable, and non-indicative of sexual orientation.
Stories with this trope can focus on romance, the exploration of sexuality and how hot it is that these two young girls are into each other. Society's reactions are also explored, as are parental and societal disapproval.
In Japan, this trope is a bit more complicated because girls having crushes on other girls is nothing that raises eyebrows in the Japanese school system. The Japanese refer to this as a Class-S Relationship, which supposedly teaches the young ladies about "real" relationships. Class-S is not supposed to become physical, and is meant to be outgrown eventually. Retaining a Class-S Relationship as one grows older is, according to some Japanese people, a sign of immaturity (e.g. immature Clingy Jealous Girls whose lesbian crush has suddenly noticed boys). Thus, it is not uncommon to find teenage girl-girl relationships in Japanese media - unless they explicitly describe themselves as lesbians, or engage in physical intimacy (at least kissing), it's not this trope but a Romantic Two-Girl Friendship instead.
Some works just pretend the girls are together and get out of it later by revealing they're not. Other times writers like to hide behind subtext in a form of Getting Crap Past the Radar.
In the eyes of fandom, a major selling point is that its principal characters are Schoolgirl Lesbians... but oops, it was just a Romantic Two-Girl Friendship after all! The viewers have been Ship Teased; and now that the writers got their attention, they can write more comfortable relationships. The lesbian subtext may not even last beyond the opening credits.
The is pretty common in Bishoujo fandom, where the Romantic Two-Girl Friendship develops for a number of unintentional factors. One could say that audience likes girls in large quantities, and seeing two girls acting cute, even at each other, is better than one. One could also point out that far more attention is put into the interaction and communication between the girls; while a relationship with a guy just "happens" with most development offscreen. And if they're the only people you see but the show still maintains particular tropes, the audience is sometimes led toward an imagined payoff that might not be there. Often, this is coupled with a young age for both characters as well as the lack of any actual 'sexuality' in either character, barring concessions, as this outcome is harder to rationalize for an adult character. Note that any romantic relationship with boys, on the other hand, is considered completely serious at any age. Yes, it's hypocritical.
Outside of canon this trope is invoked a lot especially in H-doujins due to a rather persistent tendency for male writers to be intimidated by the idea of girls who don't find them sexually attractive. This is the reason why a disturbingly large number of popular lesbian characters and characters who are popular in part because of their near lesbian tendencies are depicted in huge amounts of heterosexual porn with a random male insert or more and very frequently in the form of rape instead of with each other. Think of it somewhat as the Distaff Counterpart to All the Good Men Are Gay except this time, you can get them back on the "home team" so to speak.
This trope is also occasionally applied to male characters, usually in order to appeal to Yaoi Fangirls and an LGBT Fanbase without alienating straight viewers, in which case it's usually known as queerbaiting. Queerbaiting is usually considered quite homophobic, patronising and cynical when it's done these days, but in the past was often the only way queer creators could make media featuring characters like themselves due to the values of the time. In these cases the Slash Fic fanbase often shows up to make the Subtext text.