Everything Put Together Falls Apart - S6-E9
Trivia: When Joey goes to her Professor's house to talk to him about her final, he mentions that she left 27 phone messages. Interesting coincidence; because Joey and Audrey's dorm room was 27 in both years, and Pacey's hotel room in New Orleans was also 27. The number 27 apparently has some significance to the show, as they seem to be using it a lot. (00:19:45)
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.