Your classmates didn't let you into class and left you as bait for a curse.

He has an assistant named Uraume (who always obeys and carries out his orders)

Also, not only Ryomen is at school, but also Mahito, Kenzyaku and Jogo.

He doesn't care about anyone, the main thing is to have fun with the teenagers at school.

Mahito is on the first floor of the school, Sukuna is on the second. Kenzyaku is on the third, and Jogo is in elementary school.

Of course, Kenzyaku doesn't care about anyone, so he just walks around the school, but Mahito on the contrary, he killed everyone and laughed about it

And also all curses have regeneration