Why is Jem's decision significant to Scout? In what way do they "part ways" at this moment?

Jem is not only frightened by the pants incident, but also guilt ridden for his part in angering and lying to his father. This is the point at which Jem starts to grow up and look at things a bit differently. He's still a kid, but he's beginning to understand adult perspective and responsibility. As the younger child, Scout still doesn't understand why Jem would risk getting caught, though she will come to understand their mistreatment of Boo later in the story.