Mahou Sensou, also known as Magical Warfare, is a Japanese light novel series that was adapted into a manga series in 2013. An anime adaptation premiered in January 2014.
Takeshi Nanase is an ordinary high school boy with a somewhat dark past and, while he seems really hard working, he actually really cared about what people think of him. Due to certain circumstances, he is dating his childhood friend Kurumi Isoshima, but they are in a "fake" relationship. His life is relatively normal until the day he comes across a girl collapsed on the school campus.
The girl, in a uniform he has never seen before, is named Mui Aiba and she claims to be a magician. Mui then apologizes to Takeshi because she turned him into a magician, too, and what Takeshi once thought was just one world is actually two — the world where magicians live and the world where humans live.