Former Girls' School

Anime love setting episodes in High School for some reason. Perhaps it's purely for nostalgia for a simpler time. Perhaps it's to appeal to the demographic currently attending high school. Perhaps it's just for Teen Drama and the girls. After all, a guy falling in love with someone in a Love Triangle is a cheap and easy way to create drama.

In the latter, enter the Former Girls' School. It was open to only girls... until the protagonist steps in. Thus, it is mostly filled with female students, particularly in higher year levels. Enter the male protagonist, attending the brand new, recently converted school. Voila! You've got an easy way to justify a mostly female supporting cast.

If the One-Gender School exists for long enough, it may convert to this.

Anime and Manga

 * High School DxD
 * Prison School
 * Seitokai Yakuindomo, in which most the girls are uber-perverted and the main male of the cast is the Only Sane Man in the most literal sense.
 * The World God Only Knows provides a fitting setting for a Dating Sim master.
 * Inverted in Green Green, which is set at a Former Boys' School.

Web Comics

 * Wild's High in Girls of the Wild's has the male lead as the only boy in the school.

Real Life
""We'll have to see if his school year is full of accidental boob-grabs, tsunderes, and at least five suitors.""
 * Anime News Network reports: Life Imitate Art When Boy Enrolls in All-Girls Japanese High School. [sic]