How did Norbert get Harry Potter and his friends in trouble?

Answer 1

Harry, Ron and Hermione contacted Ron's brother Charlie and arranged for Norbert to be collected from the Astronomy tower at midnight. Draco went to visit him in the hospital wing and threatened to tell Madam Pomfrey what had really bitten him. When asked why he was there, Draco lied and said he was borrowing a book from Ron who gave him the book containing Charlie's letter.

The Astronomy tower is out of bounds expect during class times and first years were not allowed in the corridors at midnight. Harry and Hermione decided it would be okay because they had the invisibility cloak. However, they forgot to put it back on and were caught exiting the tower by Argus Filch who took them to Professor McGonagall.

McGonagall had been warned by Malfoy and had given him into trouble for being out of bed. She'd also found Neville Longbottom, who had overheard Malfoy and was trying to warn Harry. This caused all four of them to receive detention and they lost 50 points each - resulting in Gryffindor loosing 150 points in one night.

In the movie adaptation, this was changed. Draco saw Norbert through Hagrid's window and reported it to a teacher. Harry, Ron (instead of Neville) and Hermione were given detention as was Draco.