Black Gate: Kanin no Gakuen
Genres: Fantasy, Supernatural, Hentai
Type: OVA, Episodes: 2
Aired: Jul 25, 2004 to Dec 25, 2004
Studio: Studio Jam
Source: Visual novel
Rating: Rx - Hentai
Synopsis:
Narifumi Hosokawa prepares for his high school's culture festival with the Kadomori twins, Shizuku and Kasumi. While everyone is distracted, Shizuku decides to open a mysterious gate through the use of magic and travel to an alternate world. She has broken the oath of her family's heritage, as to protect the Black Gate from those who seek the enormous power that lies within the world. The remaining defenders of the Black Gate, consisting of female students and a female teacher, must travel through to the other side and return before any distruptions come to either dimension. However, Narifumi accompanies the group and Shizuku is waiting for them with some new allies. 

(Source: ANN)