I am assuming that you are asking about the incident between Hillari Kimble and Stargirl that takes place near the end of the book at the Ocotillo Ball. In that incident, Stargirl spontaneously leads a large group of students in the Bunny Hop. These students have so much fun and are so wrapped up in what they are doing that Stargirl leads the Bunny Hop line out of the Country Club and out onto the golf course. They are gone for quite awhile. In the meantime, Hillari, furious at not being the center of attention, demands that the band play "regular music" and drags her boyfriend out on the dancefloor, but to no avail. The students who have stayed behind are only concerned about Stargirl and the Bunny Hop line, and when the dancing group returns, they run out to meet them. This sets the stage for Hillari's confrontation with Stargirl. Hillari walks up to Stargirl and says, "You ruin everything", and slaps her. Hillari flinches, waiting to be struck back, but Stargirl reacts by kissing her gently on the cheek. Then she is gone, never to be seen again (Chapter 31).
Hillari does a number of mean things to Stargirl during the course of the narrative. When Stargirl first comes to Mica Area High School, Hillari spreads a rumor that the new girl is a plant, introduced by the administration to stimulate school spirit. Stargirl does not react to this or any of the other rumors circulating about her; she just goes on being herself (Chapter 2). A few days after she arrives, Hillari tells Stargirl in front of everybody that the next day is her birthday, and that Stargirl had better not sing "Happy Birthday" to her in the cafeteria, as is her habit. Stargirl says that she will not, and keeps her promise. She does, however, sing "Happy Birthday", but to Leo, whom she chooses because "he is cute", thus meeting Hillari's challenge but remaining true to her word (Chapter 6).