Towakan is a collection of stories by the creator of Joshi Kousei (High School Girls). One of the stories, "Rosehip Diary ~Whispers of Roses and Lilies(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 "soeur 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). It originally appeared in 2003 issues 1-3 of "Comic Usaman" (all except the sidestory, which was drawn specifically for this volume).
Some of the other stories in Towakan are a little more out there, so watch out for tentacle sex and whatnot if you're flipping through the book.
"Rosehip Diary" is now being scanlated by Horobi no Michi.