Demon Instructor at the Royal Academy of Magic

Demon Instructor at the Royal Academy of Magic summary:

Setsuna is a perverted instructor who is unable to invoke magic.
As a part-time lecturer at the Royal Academy of Magic, he continually tells his students to self-study.
The new students who entered the academy with high expectations are disappointed in Setsuna, and judging him to be incompetent, they challenge him to a Guess duel.
"": Will you resign if I win?
"": "I"? Surely you mean " we." All of you have been pretty annoying, so I want you all to take on me. But in return, you know what will happen if you lose, right?
Setsuna plays with the special students, who are the best in a hundred years, with perverse and astonishing magic that is unthinkable in common sense!
But behind the demon-like behavior, could there be a true purpose...?