Professor McGonagall: Nothing, I repeat, nothing gives a student the right to walk about the school at night. Therefore, as punishment for your actions, 50 points will be taken. Harry: 50? Professor McGonagall: Each. And to ensure it doesn't happen again, all four of you will receive detention. Draco Malfoy: Excuse me, professor, perhaps I heard you wrong. I thought you said the four of us. Professor McGonagall: No, you heard me correctly, Mr. Malfoy. You see, honorable as your intentions were, you too were out of bed after hours. You will join your classmates in detention.
Malfoy was under the impression that snitching on the threesome would exclude him from punishment. He was wrong.