Factual error: When Jack and Andie are talking at the dinner table about her cheating on the PSAT, Jack says, "Do you really want to get kicked out of school for cheating on something that has the world 'Practice' in front of it?" PSAT actually stands for Preliminary Scholastic Aptitude Test, not Practice SAT. This is the same mistake that Pacey made in an earlier episode.
Continuity mistake: When Pacey is nursing his black eye with a raw steak, Joey comes into Doug's apartment to talk to Pacey. Just when Pacey says, "Looking out for ya," there is a frontal shot of Joey. Her hair is bunched up considerably in the back, near her right ear. As Pacey says, "To look after said girl," the next shot of Joey shows the bunched up hair in the back suddenly gone, and her hair is smooth.
Continuity mistake: At the end of the episode, Joey and Pacey are repainting over the ruined mural with light blue paint. Joey starts to paint a small section of the wall with her roller. Dawson then comes out of a room, and glances down the hallway to see them both painting. But Joey's patch of light blue paint at this point is suddenly much larger. It doesn't seem realistic that Joey would have been able to paint that much more of the wall, in the short amount of time elapsed.
Answer: It's only explained once in the series. Jack says to Andie "I tried to start kindergarten without you", meaning Andie was smart enough to start school early.