Crossroads - S2-E2
Continuity mistake: When Joey finds Dawson at Pacey's Pier Party, it is raining heavily. Both Dawson and Joey get soaked and their hair is very wet as they reunite and kiss. At this point, it suddenly stops raining and does not rain again the rest of the time at the pier. But later, when Jen and Abby are talking behind them, both times Dawson and Joey are shown from the rear their hair is noticeably dry, wavy and styled. Later, when Dawson and Joey are talking again before she goes home, their hair is once again very wet and combed flat against their heads. Also, neither Jen or Abby's hair appeared wet at all during the party.
Answer: They are not twins (Jack is older than Andie). It is possible due to when birthdays fall in the school calender for non-twin siblings to end up in the same grade. Say that you have to be 5 on Sept 1 to start Kindergarten, you are born on Sept 2, 2000 and your little sister on Aug 30, 2001. Sept 1, 2005, you are 4 years and 364 days and can't start. So Sept 1, 2006, you are 5 years and 364 days and your sister is 5 years and 1 day, and, voila!, you're both in the same grade.