Ms. Caroline sees a "cootie" in Burris Ewell's hair. This is undoubtedly lice. She is terrified, particularly since she has very little experience with country folk. Burris has traditionally only come to school on the first day, so he exits after this embarrassment and makes comments to Ms. Caroline that are so harsh she begins to cry. While Scout does feel some degree of empathy for her teacher, she soon loses respect for her when Ms. Caroline tells her that Atticus should not read to her from the newspaper anymore. Scout also learns just how harsh and abrasive the Ewell clan can be, which foreshadows the climactic events later in the novel.
As was mentioned in the previous post, Miss Caroline witnesses a "cootie" crawl out of Burris Ewell's hair in the middle of class. Another name for a "cootie" is a louse. A louse is a parasitic insect that feeds off of a human scalp. When Miss Caroline sees the louse, she jumps and screams in terror. One of the students, Little Chuck Little, acts like a gentleman and tries his best to settle Miss Caroline down by grabbing her a cool drink of water. Scout goes on to mention that Burris Ewell was the filthiest human being she ever saw. Miss Caroline then tells Burris to go home and wash up. Burris proceeds to give her attitude and calls her several derogatory names after she threatens to call the principal on him.