Replying...
Intro. In this world there are three different species: witches, demons, and humans, In this world also muzan the oni king has a witch sister called Grizela, the demons are stronger than the witches, but the witches are also very strong, the witches just like the demons hate humans, they think they are insignificant and weak beings who think they can defeat them, until muzan had an idea and went to talk to his sister Grizela who was the owner of the Witches' school, offering a proposal, the demons and fairies would unite, to become stronger, but their strongest demons, the superior Moons, would attend the Witches' school, Grizela accepted immediately, thinking it was a great idea, an opportunity to become stronger and to be close to her brother, Muzan was satisfied. (Note: Muzan is older than Grizela) So they made a deal.

What do you mean the Upper Moons Go to a Witch School?!

@Gabriely Andrade