Class S, or "S kankei", abbreviated either as S or Esu, is an early twentieth century Japanese wasei-eigo term specifically used to refer to strong emotional bonds between schoolgirls, and a genre of girl's fiction which tells stories about the same, particularly a mutual crush between an upperclassman and an underclassman. The S is an abbreviation that can stand for "sister", "shōjo", "sex", "schön" (German: beautiful), and "escape".

Class S had links to the Takarazuka Revue, an all-women revue established in 1914, in which the stories feature male characters romancing women, with female actresses playing both the male and female roles. In this particular style of love, the women who have been influenced by Takarazuka return to their daily lives and develop crushes on their female classmates or coworkers. This type of romance was typically seen as fleeting and more of a phase in growing up rather than true homosexual behavior; as long as these relationships remained confined to adolescence they were regarded as normal, even spiritual. These relationships were common, and it has been proposed that eight out of ten schoolgirls had Class S relationships. Dōseiai was another term coined at the turn of the 20th century to describe same-sex female relationships; both of two feminine partners and of a masculine and feminine partner (also called ome). It was suggested in the popular media of the time that the Takarazuka otokoyaku (the woman playing the masculine role) caused women in Class S relationships to become ome couples (butch and femme), and persist in homosexual relationships long after it was acceptable. Jennifer Robertson sums this up in her theory, saying that "many females are attracted to the Takarazuka otokoyaku because she represents an exemplary female who can negotiate successfully both genders and their attendant roles and domains."The creation of girls' schools was very rapid at the time: by 1913 there were 213 such schools. The western novels Little Women and A Little Princess were translated into Japanese in 1906 and 1910, respectively, in order to educate the girls to become "good wives, wise mothers". However, these works also introduced western concepts of laotong, sisterhood, sentimentalism, and romance to the girls of Japan. The tomboyish Jo of Little Women particularly gave Japanese girls a different idea of adolescence. In 1936, Class S stories were banned by the Japanese government. As co-educational schools became more prominent, Class S relationships became more discreet.

An influential Class S author was Nobuko Yoshiya, a lesbian Japanese novelist active in the Taishō and Shōwa periods of Japan, who was involved in the Bluestocking feminist movement. A modern-day yuri light novel series which strongly borrows from the Class S genre is Maria-sama ga Miteru. It is considered to be a modern equivalent to Yoshiya's Hana monogatari.