Former Girls' School

A male protagonist attends a middle or high school that until recently was open only to girls and still has mostly female students, thereby justifying the mostly female supporting cast.

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 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]