Oshiego wa Mahou Shoujo ~Chinami ni Ore wa Kyoushi de Kaijin~

There are allies of justice, operating as magical girls, Akasaka Kanon and Aono Riri. These two are protecting good citizens, while fighting an evil organization planning to overthrow the world. They are extraordinary beings—all are grateful to them, all envy them. One time in school, which they attended unbeknownst to the public, came a new teacher. This teacher was kind to every single student, quickly solved all problems and made his lessons interesting. A wonderful teacher... It was that all thought about him. And yet! In reality, he is an elite monster who has come to plunge the magical girls into darkness! He makes use of his lessons to brainwash the magical girls and, furthermore, in the night, when he turns back into monster form, disgraces the souls and bodies of the magical girls using cowardly tricks. And they are changing into dark magical girls who are drowned in pleasure and evil... [Translated from DLsite]