Zodiarc

It's a world of swords, magic and modern science. Main character is a teacher at a magic school in a city called Arcana. But he is only 19 years old, so students make fun of his blunt attitude and strange behavior. Disappearances of people start to happen everywhere. One day he meets a woman at night, and she turns out to be a demon. In order to regain quiet everyday life main character decides to defeat Veronica, the strongest demon called Zodiarc.

[From vndbreview]