This is a good question. Hard times await Scout in school. The reason for this is because some children in school are making fun of Scout by saying that Atticus defends "niggers." In other words, the racism of Maycomb comes out, and it is affecting Scout at school.
So when Scout faces Cecil, and she wants Cecil to take it back, Cecil does not. In fact, Cecil provoke her to a fight. At this point, Scout resists, even though she had her fists clenched. Here is how Scout describes the situation:
I drew a bead on him, remembered what Atticus had said, then dropped my fists and walked away, “Scout’s a cow-ward!” ringing in my ears. It was the first time I ever walked away from a fight.
Scout was able to do this, because she remembered the promise that she made with Atticus. Furthermore, she remembered that Atticus asked Jem and her to do so little that she did not want to let him down. When she walked away, she felt noble. Here is what she says:
Somehow, if I fought Cecil I would let Atticus down. Atticus so rarely asked Jem and me to do something for him, I could take being called a coward for him. I felt extremely noble for having remembered, and remained noble for three weeks. Then Christmas came and disaster struck.