Maria-sama ga Miteru

The setting is Lillian Academy, a private Catholic all-girls school where the student council is known as the Yamayurikai (Mountain Lily Council) and younger students are taken under the wing of older students through a system known as the "soeur system" ("soeur" is French for "sister"). The main character, Yumi, who is basically a nobody, suddenly finds herself interacting with the members of the Yamayurikai and their soeurs, most notably her crush, the talented, beautiful, and withdrawn Sachiko...

A spin-off series of Maria-sama ga Miteru which focuses on Yumi's younger brother Yuki and his schoolmates at Hanadera Academy. (Source: Maria-sama ga Miteru Wiki)