Factual error: When Jack and Andie are talking at the dinner table about her cheating on the PSAT, Jack says, "Do you really want to get kicked out of school for cheating on something that has the world 'Practice' in front of it?" PSAT actually stands for Preliminary Scholastic Aptitude Test, not Practice SAT. This is the same mistake that Pacey made in an earlier episode.
Continuity mistake: When Pacey is nursing his black eye with a raw steak, Joey comes into Doug's apartment to talk to Pacey. Just when Pacey says, "Looking out for ya," there is a frontal shot of Joey. Her hair is bunched up considerably in the back, near her right ear. As Pacey says, "To look after said girl," the next shot of Joey shows the bunched up hair in the back suddenly gone, and her hair is smooth.
Continuity mistake: At the end of the episode, Joey and Pacey are repainting over the ruined mural with light blue paint. Joey starts to paint a small section of the wall with her roller. Dawson then comes out of a room, and glances down the hallway to see them both painting. But Joey's patch of light blue paint at this point is suddenly much larger. It doesn't seem realistic that Joey would have been able to paint that much more of the wall, in the short amount of time elapsed.
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.