Green Green (2001)

Green Green is an erotic visual novel developed and published by Groover that takes place in an isolated and nondescript boarding school known as Kanenone Gakuen that prides itself on the tradition of men growing amongst other men without any external influences to disturb them. Until one day, when the current administration decided to consider making the academy co-ed, and in order to see how the change would affect the student body, first and second-year girls were brought to live on school grounds for one month. While you, Yuusuke Takasaki, take the change in stride, your three hormone-driven friends see this as the true beginning of their youth.