Continuity mistake: When Joey and Pacey are sitting on the bed talking during the party, there is a shot where Joey is looking to her right, directly at Pacey. The next shot of her, she is suddenly looking down and completely away from Pacey. The transition is too fast to be realistic. (00:16:55)
Continuity mistake: At the end, Pacey carries Joey upstairs and lays her on his bed because she is drunk. In one shot of Joey, her hands are up by her face but the right shoulder of her sweater is slipped off her shoulder, revealing the strap of her shirt underneath. The next shot of Joey, her left hand is still up and her right hand is down, but the right shoulder of her sweater is back up in place. It was too fast for her to fix the sweater herself, and her left hand hasn't moved. (00:40:25)
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.