The Song Remains the Same (2) - S6-E2
Revealing mistake: When Jen and CJ are talking at Boston Bay University, and CJ mentions 'The Stand'; Jen suddenly gets up to leave thinking he is referring to religion. As she turns to walk away; you can see for a brief moment the real tattoo on the lower back of Michelle Williams, in the slight gap between her shirt and pants.
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.