Snape normally enjoyed tyrannizing his student assistant. But Percy Weasley was not giving satisfaction.
When Snape ordered him to mark eighty-three essays, Percy returned them by 8 AM, with impressively scathing comments in the margins.
Percy never erred, never complained, and treated the students so severely that Snape had no reason to hector him.
When Snape assigned his other duties, Percy blanched and Snape smiled nastily. But even on his knees in the dungeon, Percy's performance was faultless.
Snape scowled. Then Percy finished.
When he could breath again, Snape supposed he could stand Percy for a few more months.