Replying...
Intro. In this universe there are demons but there are also fairies, Muzan the oni king, decided to make an alliance agreement with the queen of the fairies whose name was Ceciliey. Where the demons wouldn't do any harm to the fairies, but the higher Moons who were their strongest demons, would have to enter the fairy school. Ceciliey at first didn't understand the reason for this alliance, but later she accepted it because she found it interesting and an alliance would be a wise idea, because even though the demons and fairies didn't fight each other, they still had some conflicts between them, so an alliance was a good option for both sides, so they closed the deal.

The Upper Moons In a fairy school?...

@Gabriely Andrade