What is one example of tone used in the novel?

Lee's tone towards the educational system is negative and critical.  Consider this quote:

"The Dewey Decimal System consisted, in part, of Miss Caroline waving cards at us on which were printed "the," "cat," "rat," "man," and "you." No commend seemed to be expected of us, and the class received these impressionistic revelations in silence. I was bored, so I began a letter to Dill. Miss Caroline caught me writing and told me to tell my father to stop teaching no."

That Scout, the narrator, has misunderstood the Dewey Decimal System and that she presents Miss Caroline as naive and incompetent demonstrates Lee's criticism.