"Harry!" Neville burst out, the moment he saw the other two. "I was trying to find you to warn you, I heard Malfoy saying he was going to catch you, he said you had a drag ––"
Harry shook his head violently to shut Neville up, but Professor McGonagall had seen. She looked more likely to breathe fire than Norbert as she towered over the three of them.
"I would never have believed it of any of you. Mr. Filch says you were up in the astronomy tower. It's one o'clock in the morning. Explain yourselves." (Harry Potter and the Sorcerer's Stone)
What does ‘it’ mean?