In Scout's first grade class at school, a conflict arises between the teacher, Miss Caroline, and one of the students, Burris Ewell. Miss Caroline spots a louse on Burris and freaks out. Another student, Little Chuck Little, who is a polite and brave kid, tries to solve the problem by calming both of them down. When Burris gets angrier at Miss Caroline for asking him to sit down, Little Chuck Little tells her to let him go. Burris Ewell leaves the class while shouting rude insults at Miss Caroline.
The Ewell kids (there are many of them) only go to school one day out of the year. The only reason they go on that one day is because the truant officer forces them to. The Ewells are lower-class white folks with very little education.
That night, Scout asks her father Atticus if she can skip school like the Ewells do, but Atticus refuses to let her. He tries to teach Scout a lesson about walking around in another person’s skin. Atticus and Scout do reach one compromise: if she will go to school, they will continue reading at night. As an aside, he asks her not to mention their reading at school. This shows that although the Finches are not extremely wealthy, they have education on their side, which makes them more "middle class" than the "white trash" Ewells. This socioeconomic conflict will come into play later in the novel, when Tom Robinson, a black man, is on trial so supposedly raping an Ewell daughter. (a fabricated story by the Ewells). The conflicts are between the "middle class" whites (Atticus is the lawyer representing Tom), the "lower class" whites, and the black community.
This scene in Chapter 3 introduces the Ewell family by framing them as people who don't quite "fit" in society because of their lack of education and socioeconomic status, but also sets it up to show how even "lower class" white people are not systemically oppressed as deeply as black people are.