乙女はお姉さまに恋してる ～2人のエルダー～ THE ANIMATION

Two years after Mizuho's graduation... Mikado Chihaya was truant at his school because of harassment in the school and of a distrust in men. His mother cannot bear to watch him be in trouble, so she makes him transfer to Seiou Girls' School, her old school. He dresses as a girl and introduces himself as Kisakinomiya Chihaya. Chihaya meets Nanahara Kaoruko, who once rescued him from a playboy, and he begins to live in the dormitory building. He becomes the focus of public attention due to his attractive face and figure, and due to high grades at studies.

(Source: Wikipedia)