The Importance of Not Being Too Earnest - S6-E3
Continuity mistake: When Joey accesses her email address book to send an email to Dawson, she inadvertently hits the address above his, sending her email to the entire Worthington campus. Dawson's email address in Joey's address book is DawsonLeery@worthington.edu. Dawson does not go to Worthington and would not have a campus email account.
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.