Onii-chan Teacher ~Himitsu no Jugyou o Kibou Shimasu!~

Shuugo grew up in a remote snowy area. He left his village to chase his dream of becoming a teacher and got a job at a city in the same prefecture. However, he was temporarily assigned as the lone teacher and administrator of the branch school until the snow melted in spring. There were only four students at the school: Kanae, Ruri, Ai and Tsubomi. They were all his imoutos and they all loved him very much. So begins his strange dual life as their teacher during the day and as their older brother at night.

[From Hau~ Omochikaeri!]