The Kids Are Alright (1) - S6-E1
Plot hole: When Joey walks into her class and sits down, her cell phone rings and her Professor answers it. As he listens to Audrey on the other end, he says very quickly that he got the message. But when he relays the entire message to Joey after he hangs up, the message is much longer. There is no way he could have possibly gotten that long a message in the very short time he was talking to Audrey and said 'he got it'.
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.