Towa-kan

Towakan is a collection of stories. One of the stories, "Rosehip Diary - Whispers of Roses and Lilles (Yuri)," takes place at a girls' school and is a bit like an adult version of Marimite-- the rhythmic gymnastics club has a system in which second-year students mentor first-year students. The twist in this "sœur system" (though it isn't called that-- the underclassmen are called "partners" or "escorts") is that the mentor also has sex with her mentee.
The main character of "Rosehip Diary," Asou Chitose, has just entered her second year at the academy and is about to take on a mentee. But not only is her new mentee, Kuon Yuki, nothing like she expected, she's a bit of a handful! She's also already started falling for Chitose...
"Rosehip Diary" is three chapters long, plus a sidestory focusing on Kuon and Yukari (a friend of Chitose's).