Season 3, Episode 18: Separate Vocations
Marge: Bart's grades are up a little this term. But Lisa's are way down.
Homer: Oh, why do we always have to have one good kid and one lousy kid? Why can't both our kids be good?
Marge: We have three kids, Homer.
Homer: Marge, the dog doesn't count as a kid.
Marge: No, I mean Maggie.
Homer: Oh, yeah.
Principal Skinner: Some sick individual has stolen every "Teacher's Edition!"
Teacher: What do we do?
Mrs. Krabappel: Declare a snow day!
Teacher #2: Does anyone know the multiplication table?
Lisa: Bart, why did you take the blame?
Bart: Because I didn't want you to wreck your life. You got the brains and the talent to go as far as you want. And when you do, I'll be right there to borrow money.