That Was Then (aka Before and After) - S6-E16
Revealing mistake: In the very beginning of the episode, Pacey is watching TV and gets up to look out the window towards Hell's Kitchen. As he moves back from the window, he accidently bumps the kitchen counter/cabinets. An entire section of 'counter/cabinet' moves noticeably; revealing it to be a cheap, flimsily-built fake kitchen counter/cabinet instead of a more substantial, real one. This was obviously just part of a set.
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.