The Scare (aka Friday the 13th) - S1-E11
Trivia: In one scene, Jen is talking to a stranger on the phone, who at one point says, "How's your grandfather?" This prompts Jen to check on her sick grandfather. This line was a reference made by co-creator, Kevin Williamson, to the original "When a Stranger Calls." Williamson used films like "When a Stranger Calls" and "Halloween" for the basis of his film "Scream," which happens to be the beginning premise of this episode.
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.