So, at some point the headmaster and the policeman appear and they find blood. The horrified headmaster covers his mouth with a hand. Katherine has just killed and beheaded the sisters.
But then film goes back to show what happened.
Katherine (Kiernan Shipka) made a call and the devil spoke to her through the phone: 'kill all the cunts'.
Then, we see a second call. This time she's not the one doing it: she is being called. She's wearing her reptilian hairdo.
And after that, she goes down to the furnace room. But that's intercut with her killing the sisters, going upstairs, etc.
The demonic voice is heard, buried in the soundtrack.
What is the voice saying? I can hear 'go down...' but not the rest.