Houkago no Magus

Houkago no Magus (放課後の魔術師〈メイガス〉) is a light novel series by Tsukasa Tsuchiya with illustrations by Hauraki Fuyuno.

Plot
One morning, Harihata Haruka meets a boy, Akitsu Aki. She thought he was a transfer student, so she treated him kindly, but she was shocked that when she found out he was 17 years old and was her homeroom teacher. Meanwhile, with Aki's appearance, a mysterious phenomenon starts to occur simultaneously. It turn out to be, Aki was a logical magician that creates an all possible creation, twisting a phenomenon. He had a hidden intention that defeat enemies that lurking around in school. Haruka was aware of Aki's suspicious behavior and she tried to get close to him but her act was a stepping into world of logical magician.