Sleeping Arrangements - S5-E12
Continuity mistake: When Joey is talking to Elliot in the classroom, she bends over her desk and gathers all her hair in front of her left shoulder. But in the next shot of her, you can see that her hair is suddenly in front of both shoulders. The transition was too quick to be realistic. (00:06:45)
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.