Orrr maybe sensei is half demon/yokai. The principle did mention some demons/yokai true forms resemble humans and thats why they have integrated into society and some people still suspect certain breeds of yokai(which they think are extinct) could be actually hidden amongst the higher class humans.
My guess is maybe sensei's half and thats why he doesnt have a demonic form hence no needing to suppress it and never came to know he has demon abilities. Maybe if he encounters a moment which pushes his limit and he could awaken?
Or they met before ┑( ̄Д  ̄)┍ considering demons live longer. Knowing this author, we can except a good explanation and storyline.
It seems like to me those two are maybe mates or soul mates???? Can this happening between demons and humans????(〜 ̄△ ̄)〜