Sister Princess

A top student named Wataru Minakami failed his high school entrance exam due to a computer error. He later learns he was accepted into Stargazer Hill Academy on a place known as Promised Island. At his father's request, Wataru is taken to the island, where he is immediately approached by a group of twelve girls who claim to be his younger sisters. As he spends time with them, a larger mystery emerges regarding their presence on the island.