Why does Scout forget what Atticus told her about fighting?

A boy at school, Cecil Jacobs, teases Scout, saying that her father "defends niggers". Scout will not accept insults about her father and fights Cecil. Scout is having trouble dealing with the ugly prejudice reared against her father because of the trial.

Cecil Jacobs was not nice to scout, and saying that scouts dad was "defending niggers," and scout didn't like that at all. So scout beat Cecil Jacobs up forgetting what attics had said.