Continuity mistake: When Pacey is driving Joey back from Boston in the morning, they start talking about what happened. Every shot of Pacey or Joey from either side shows a heavily wooded forest (mostly pine trees it looks like) in the background. But when Pacey abruptly veers off the road to the right and parks; suddenly the landscape on Joey's side is barren with a huge lake, and no forest or trees except a few they just passed on the curve. The pine tree footage was probably added in to make Wilmington, NC look more like Capeside, Mass.
Revealing mistake: When Joey and Pacey are talking in the car as Pacey drives them to the Train Station, you can see at least 3 lanes full of cars and a great deal of traffic behind them. But when Pacey suddenly turns right, off the main road and into the entrance to the Train Station; you can see that suddenly the main road he has just turned off of is almost completely empty of any traffic in either direction, except a truck that drives by at the last minute. It also appears to be a smaller road. Apparently the scenes with their dialogue in the car were shot with background video that wasn't the actual location.
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.