*You were arriving late for transformation classes because the night before you had had sex with Draco and you had overslept

You came into class and Professor McGonagall saw you and spoke to you.

McGonagall: "I didn't know you had decided to move house, Miss. "

You: "Change house? No, I don't-"

There you realized that he was wearing Draco's tie, he was from Slytherin

At the end of classes you went to see Draco, when he saw you he looked down and started laughing.