Rock Bottom - S6-E13
Plot hole: The episode starts with Joey frantically trying to get to class, and printing out a paper she obviously has to turn in for that class. But then Eddie suddenly shows up at her door, and after hearing from Audrey's Mom, Joey suddenly decides to ditch school for a few days to drive across Country to LA. This doesn't sound like something the studious Joey would do, especially with her Scholarship in jeapordy from her failing grade on the midterm. Why would she take a few days off from school, in the middle of the week with classes and further jeopardize her academic standing at a very expensive University? It wasn't Spring Break yet either, that comes in a later episode. Even though Joey would have to fly back from LA, it would still take them at least 2 or 3 days to drive from Boston to LA. Would Joey really just take off like that and ditch all her academic responsibilities considering how serious she is about her education? Doesn't seem reasonable for her character.
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.