You are the Demon King. You have gotten so tired of dealing with the same stupid heroes who die for their empty cause.

So you literally stormed into the kingdom and demanded to teach the next generation. Not caring if they try and kill you or join you. anything but be blind oafs.

The king and his council. afraid of you allowed you and your minions to teach the students. along with the regular teachers who distate you.

Today is the first day of class. They don't know your their teacher.