When does David enjoy his lessons?

It's fair to say that the schooling which David receives is something of a mixed bag. At Salem House, he's placed under the authority of its unspeakable headmaster, Mr. Creakle, who gains an almost sadistic pleasure in administering harsh physical punishment to the boys. Although David makes friends with James Steerforth—even though he'll come to regret it—and the much nicer Tommy Traddles, his experiences at Salem House are not very happy on the whole.
Things couldn't be more different at Dr. Strong's school in Canterbury, where David's kind aunt Betsey Trotwood sends him after his escape from the hell-hole of the bottling plant. As both a teacher and a human being, Dr. Strong is a world removed from the vicious Mr. Creakle. He's an unfailingly kind, decent, humane individual whose admirable personality helps to create an educational environment in which David is soon able to thrive.