Why does Burris call Miss Caroline a slut?

Burris, he of the notorious Ewell clan, has turned up for the first (and for him, last) day at school. Personal hygiene's not exactly a strong point of the Ewells, and Burris has inherited his folks' less-than-scrupulous attention to matters of appearance. That's not all he's inherited. His unkempt hair plays host to a thriving ecosystem of lice, or "cooties" in the local vernacular. One member of this little colony crawls out of Burris' tangled thicket and gives poor Miss Caroline a fair old fright. Somewhat flustered, she orders Burris to go home and wash his hair with lye soap and kerosene.
Burris's annual learning experience comes to an abrupt end. He spits at Miss Caroline and calls her a "snot-nosed slut of a teacher" before storming out of class. Burris is clearly a chip off the old block. He has as much respect for people, especially women, as his thoroughly obnoxious old man. To call a respectable woman a "slut" in Maycomb is just not the done thing. By using this word, Burris confirms the status of the Ewell clan as social outcasts, "white trash" unfit to be admitted into polite society.