In "To Kill a Mockingbird", why does Scout get into another fight at school?

In Chapter 9 of "To Kill a Mockingbird," Scout becomes angered at school by Cecil Jacobs's remark that her father defends African-Americans.  Scout denies this, and later asks her father if he does defend "Negroes."  He tells her that he cannot hold up his head if he does not defend Tom Robinson, explaining why and instructing Scout to "fight with your head." And, she is to remember that this time they are not fighting Yankees, but their "friends."

The next day, Scout tries to keep her father's words in mind, but she insists that Cecil retract his words.  He refuses and insults her father further, but Scout does not hit him; she feels noble.  However, later at Christmas dinner, her cousin Francis insults Atticus in the same manner as Cecil.  Scout is incensed that Francis would say this and "collars" him and does not allow him out of the kitchen.  Then she splits her knuckle on his front teeth.  But, because she has called Francis a name, Scout is punished with a "licking" from Uncle Jack.