Although many of Scout's classmates come from poor families and are repeating first grade, Burris Ewell is described as being the filthiest human she has ever seen. Unlike the majority of Scout's classmates, Burris Ewell is a rude, disrespectful child who insults Miss Caroline after she threatens to call the principal. Burris also refuses to attend school for the remainder of the year after the first day. Scout's other classmates behave admirably and assist Miss Caroline after Burris Ewell makes her cry. The other children throughout the novel are relatively well-behaved individuals who were raised to treat authority figures with respect. Harper Lee introduces Burris in order to characterize and provide background information for the despicable Ewell family. The central conflict throughout the novel involves an innocent African American man who is accused of assaulting and raping Burris's sister, Mayella Ewell. Burris's offensive behavior illustrates his negative upbringing and sheds light on the disgraceful ways of the Ewell family.
The description of Burris as crude and unclean foreshadows the later events of the novel when Tom Robinson is accused of raping Mayella Ewell. By introducing Burris, the Ewell family's history is also introduced, so we aren't shocked by what happens in the trial. Knowing this background allows us to feel some pity for Mayella when she later testifies, but we feel nothing but disgust toward Bob Ewell, knowing that he allows his children to live in such a way. This early information makes it even more sickening that Tom Robinson is found guilty.
Burris Ewell is a very nasty, ungroomed boy. Most other student's don't go near him due to his filthiness. Other kid's also go to school other then just the first day. This description is used so you can get a feel for the Ewell family tradition of nasty.