What could Scout know how to do that her teacher didn't like?

Miss. Caroline didn't appreciate Scout's ability to read and discouraged her from reading with her father. 

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.