During class hagrid you stood up and heard Malfoy taunting you then you said "shut up Malfoy" then Malfoy whistled and looked at you his eyes were looking up and down While biting his lip he also whistled, he knows that you are afraid of Dementors he found out earlier during Mr. Lupin's lesson, then he said

D-d-dementor! Dementor!

You immediately looked back and it turned out that he was fooling you and then he put on his cloak hat and demonstrated the Dementor's movements