Black Gate: Kanin no Gakuen

Synopsis

Narifumi Hosokawa is getting ready for his high school's culture festival alongside the Kadomori twins, Shizuku and Kasumi. When everyone's attention is elsewhere, Shizuku uses magic to open a mysterious gate and step into an alternate world. In doing so, she violates the oath of her family's legacy, which exists to safeguard the Black Gate from those who desire the immense power hidden within that realm. The remaining protectors of the Black Gate—a group made up of female students and one female teacher—must journey through to the other side and come back before any disruptions affect either dimension. Narifumi goes with them, but Shizuku is already there, waiting with some new allies.