She's in it for a good amount considering she was just a kid in the film, most of the time it is Sarah Jessica or Helen Hunt though; Shannen plays the little sister to the guy Sarah Jessica's character likes. She does have some very cute parts though, however if you're hoping to see her in every other scene she won't be there....but like I said it's a fair amount for her age and the movie's pretty good, definitely a teen flick really with it mostly being about a dance competition, but you might like it.
Hope this helps.