Why is Burris Ewell unusual?

Burris Ewell is unusual for being extraordinarily filthy and for having lice in his hair; he also speaks to the teacher with much disrespect. Each year he only comes to school on the first day. Perhaps his most inappropriate behavior is his aggression toward Miss Caroline.
When one of the students points out that Burris has "cooties," Miss Caroline notices Burris for the first time. She advises Burris on how to remove lice from his hair; he asks her "What fer, missus?" Then, Miss Caroline tells Burris to bathe before he returns to school, then:

The boy laughed rudely. "You ain't sendin' me home, missus, I was on the verge of leavin'—I done done my time for this year."

Miss Caroline is perplexed by Burris's claim that he only comes one day each year. An older member of the class informs his teacher that the Ewells are always truant. They only attend the first day because the truancy officer "threatens 'em with the sheriff." However, she has despaired of keeping them in school during the year. As Burris continues to be insulting, Miss Caroline tells him to sit down. "You try and make me, missus," Burris retorts. Little Chuck Little suggests that his teacher let him go. "He's a mean one, a hard-down mean one. He's liable to start somethin', and there's some little folks here." Burris does leave, but before he goes, he insults Miss Caroline upon departing.