Lost Weekend - S5-E2
Continuity mistake: At the end of the episode, the airport staff member (visible in the background) announces that the final boarding for Dawson's flight is at gate C-3. As the scene ends, the camera pulls away to reveal the gate number to be B-1.
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.