This is my favorite episode of The Simpsons:

This is such an unashamed morality play that if it were less good it might be annoying. But the moral message of the importance of studying and working hard is driven home with such a skill and thorough characterization that it remains entertaining throughout even as it delivers an insightful and poignant lesson.
Bart is falling further and further behind at school and is threatened with having to repeat the fourth grade. This threat is enough to scare him into action and he enlists Martin's help in studying.
Unable to concentrate on revision, Bart slaps himself around the face, muttering: "You wanna be held back a grade? Concentrate, man!" There is a sudden jump cut to the next day, as the class are handing in their completed test papers - and Bart is still slapping himself. Even in cartoon form the sight of Bart slapping himself to try and force himself to focus is a really effective character moment. He is clearly trying his hardest to succeed and when his tears of failure come they are understandable and sad.
By the start of the second series, of which this was the first episode, Bart had become something of a counter-culture hero. But rather than capitalize on the character's punk reputation, the show's writers decided to show that, deep down, he was scared of failure.
At every single step the writing focuses clearly and absolutely on Bart's situation and state of mind. Every avenue and possibility is explored to build up Bart's state of desperation. This was, and still is, a bold and daring moral for a cartoon show, and the emotional payoff and the relevancy of this hard lesson (Even when you try hardest, it still might not be enough-and it's okay!) are why this episode represents the very best The Simpsons has to offer.

Favorite Moment: (Bart has just given a poor book report on Treasure Island.)
Mrs. Krabappel: Bart, did you read the book?
Bart: , I am insulted. Is this a book report or a witch hunt?
Mrs. Krabappel: Then perhaps you'd like to tell us the name of the pirate.
Bart's Brain: Blackbeard. Captain Nemo. Captain Hook. Long John Silver. Peg Leg Pete. Bluebeard.
Bart: Bluebeard?