In the book Stargirl, why does Hillari Kimble say Stargirl isn't real?

Jerry Spinelli's novel, Stargirl, tells the story of an unusual girl named Susan, though she prefers to go by "Stargirl." A new student at Mica Area High School, Stargirl's eccentric clothing and odd behavior garners all kinds of attention and affection, especially that of the narrator, Leo. 
 
While almost everyone is curious about Stargirl, Hillari Kimble, one of the more popular girls in the school, decides that Stargirl is a fake and doesn't give her much of a chance. Hillari then tells her fellow students that Stargirl isn't real, claiming she's an actress hired by the administration in order to raise morale and increase school spirit. 
 
On page 4, Leo describes the way Stargirl plays her ukelele and sings in front of the entire cafeteria on her first day of school. He says, "Heads swung, eyes followed her, and mouths hung open. Disbelief" (Spinelli, 4). Although it's never actually stated, it's likely that Hillari says that Stargirl isn't real simply because she's jealous of her.
 
Due to her popularity, Hillari is used to being the center of attention at school. So when Stargirl arrives and everyone seems to want to know more about her, Hillari writes her off as a fake in order to reclaim her status.