Based on the Shin Megami Tensei If videogame.
The setting is Karukozaka High School, which is mysteriously sucked into a void and emerges in the realm of Makai, a world of demons. Most of its students and school employees are trapped in the other world along with it.

Six months ago, Karukozaka High School was sucked into the demon world, and in the ensuing chaos, only two survivors emerged: Nobu, who defeated the Demon Lord that took over the school, and Yumi, who possesses latent demon powers of her own.
Although they're both trying to put the incident behind them, the gateway to the Underworld remains open, and soon Nobu finds himself battling demons right on the streets of Tokyo. The reason for this infestation is simple: a secret organization is trying to establish a Demon Kingdom on Earth by summoning demons through the gateway.
Nobu and Yumi are the only people to have been to the Underworld and survived, so they may have the power to stop the infestation but before that can even happen, they'll have to find each other first.