The Simpsons season 21 episode 14
Episode title: Postcards from the Wedge
When Bart fails to turn in his homework, Mrs. Krabappel sends a letter home about Bart's behaviour. Despite Bart's best efforts to intercept it, Homer reads the letter, and he and Marge visit Principal Skinner for a parent-teacher conference. Furious, Homer punishes him, but Marge takes a more sympathetic approach. When Bart realizes he can pit Homer and Marge against each other to his benefit, his scheming reaches new heights.