Why does Scout attack little Walter Cunningham at recess?

During lunchtime on Scout's first day of school, Miss Caroline notices that Walter Cunningham Jr. does not have a packed lunch or money to buy the school's food. When she attempts to give Walter a quarter to buy lunch, he refuses by simply shaking his head. Walter Cunningham Jr. is a shy boy from a poor family, and he does not feel comfortable explaining to Miss Caroline why he refuses to accept her quarter. Instead of politely telling Miss Caroline that he cannot pay her back, Walter continues to shake his head. Scout then intervenes on Walter's behalf and tells her teacher, "Miss Caroline, he’s a Cunningham" (Lee, 20). Scout mentions that she thought she had made herself perfectly clear and does not take into consideration that Miss Caroline knows nothing about Walter's family. Scout ends up getting frustrated that Miss Caroline cannot comprehend Walter's situation and ends up saying,

You’re shamin‘ him, Miss Caroline. Walter hasn’t got a quarter at home to bring you, and you can’t use any stovewood (Lee, 22).

Miss Caroline responds by punishing Scout for her attitude and makes her stand in the corner. Later on in the day, Scout ends up attacking Walter Cunningham Jr. on the playground during recess because her involvement in his "dietary affairs" made her "start off on the wrong foot." Scout is upset that she was punished for attempting to explain Walter's situation and she feels like it was Walter's fault that Miss Caroline made her stand in the corner of the room.