Black Lily to Sayuri-chan

I admit that I was on the verge of dropping this manga since it felt like one of those already-known/read school shoujo manga where you can name all the -dere archetypes. Notwithstanding that, at a certain point something sparked my interest and the story changed. I was even moved by the end.
Yuri, a girl who was raised in a strong old-school-like island, is enrolled in an all-girl school by her parents (who own a dojo). Saki (Yuri's mother) was an alumni of such institution and trusted that all the values and principles by which Yuri standed would be supported and enforced.
However, when Yuri enters the school dorm, she faces the hard truth starting with an encounter in the hallways: everybody is homosexual or experiencing sapphic love. Her own room-mate, Lily. (who happens to be the chairwoman of the school) has even a Mistress-pet relationship with a senpai, Sakura.
Their first meeting is definitely one of the worst ever, but little by little Yuri and Lily happen to start a relationship based on the will of the former to "redeem" and "save" traditions of a school which is now ruled by "freedom". So, since she is free to choose, Yuri shouts at Lily that they will be lovers in order to prove what traditions and normality are.
What lies beneath Lily's libertarian behaviour? Is it merely a physical experiment?

Read and find out their story step by step.

Spoiler alert: nothing is never as it seems.