Valentine's Day Massacre - S3-E14
Revealing mistake: When Jen goes to visit Henry at the hospital, he is eating a small individual-sized Jello. There is another identical Jello on the same tray, in front of Jen when she sits down to talk to Henry. You can see that the corner of the Jello lid has already been pulled up rather noticeably. This is probably to make it easier for Michelle Williams, the actress to be able to open the Jello quickly during her scene.
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.