Why does Scout's ability to read and write annoy her teacher, Miss Caroline?

According to Miss. Caroline, Scout shouldn't be able to read.... and anyone who might have taught her did it wrong. She tells Scout not to let her father teach her anymore, and that she would try to undo the damage. It is the school system's job to teach reading....

I suppose she chose me because she knew my name; as I read the alphabet a faint line appeared between her eyebrows, and after making me read most of My First Reader and the stock-market quotations from The Mobile Register aloud, she discovered that I was literate and looked at me with more than faint distaste. Miss Caroline told me to tell my father not to teach me any more, it would interfere with my reading.

“Now you tell your father not to teach you any more. It’s best to begin reading with a fresh mind. You tell him I’ll take over from here and try to undo the damage-”