Why does Atticus tell them to forget it in To Kill a Mockingbird?

Another example of Atticus suggesting that Scout forget
about advice she has been given in To Kill a Mockingbird comes
after Miss Caroline has told her that

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"Your father does not know how to
teach."

Miss Caroline
believes that the reading sessions that Atticus and Scout so enjoy should be ended. When
Scout tells Atticus about this, he asks her if she knows what a "compromise" is. He then
explains that if she will agree to continue going to school, they will continue to read
each night--but without informing Miss Caroline of their agreement. The teacher might
not approve of their "activities," he tells Scout, so it will be just their little
secret.