I shall expect you and the Slytherins in the Great Hall in…

““I shall expect you and the Slytherins in the Great Hall in twenty minutes, also,” said Professor McGonagall. “If you wish to leave with your students, we shall not stop you. But if any of you attempt to sabotage our resistance or take up arms against us within this castle, then, Horace, we duel to kill.” “Minerva!” he said, aghast. “The time has come for Slytherin House to decide upon its loyalties,” interrupted Professor McGonagall. “Go and wake your students, Horace.” Harry did not stay to watch Slughorn splutter: He and Luna ran after Professor McGonagall, who had taken up a position in the middle of the corridor and raised her wand. “ Piertotum --oh, for heaven’s sake, Filch, not now --” The aged caretaker had just come hobbling into view, shouting, “Students out of bed! Students in the corridors!” “They’re supposed to be, you blithering idiot!” shouted McGonagall. “Now go and do something constructive! Find Peeves!” “P-Peeves?” stammered Filch as though he had never heard the name before. “Yes, Peeves , you fool, Peeves ! Haven’t you been complaining about him for a quarter of a century? Go and fetch him, at once!” Filch evidently thought Professor McGonagall had taken leave of her senses, but hobbled away, hunch-shouldered, muttering under his breath. “And now-- Piertotum Locomotor! ” cried Professor McGonagall. And all along the corridor the statues and suits of armor jumped down from their plinths, and from the echoing crashes from the floors above and below, Harry knew that their fellows throughout the castle had done the same. “Hogwarts is threatened!” shouted Professor McGonagall. “Man the boundaries, protect us, do your duty to our school!””

— J.K. Rowling