Ashley Tisdale has a small role in the movie as a high school student named Kim. In the middle act of the film, motivational speaker Jim Cunningham (Patrick Swayze) is speaking at an assembly at Donnie's high school during which students are allowed to come up to a microphone and ask Jim questions or voice concerns to which he will give advice.
Kim comes up to the microphone and expresses concern for her stepsister, whom she worries eats too much. Her stepsister is not too pleased with this, telling her, "Shut up, Kim" from the audience. Following this, Jim invites Kim onstage as he gives an empty response about ego and inner beauty.
It's a very small scene in the film, so it would be easy to miss Tisdale, especially because this role came before her big break on "The Suite Life of Zack & Cody." She also had dark brunette hair in the film, which is very different from her typically blond locks. Tisdale is not the only teen sitcom star to make an appearance in the scene, however. Jerry Trainor of "iCarly" fame comes up to the mic right after Kim. Throw in Seth Rogen's character as well and you have a whole new wave of talent that was just waiting to make it big time.