Why does Scout rub Walter Cunningham's nose in the dirt?

This is because he made her "start off on the wrong foot
with her first grade teacher, Miss Caroline.

Scout got in
trouble with Miss Caroline partly because she was trying to explain to the teacher why
Walter did not have any lunch.  Scout was trying to explain to the teacher that Walter
was too poor to have lunch.  Miss Caroline got mad at Scout because of
this.

Miss Caroline is mad because (in my opinion) Scout
makes her feel like she does not know what she is doing on her first day at this new
school.