In chapter 2 of To Kill A Mockingbird, why does Scout stand up for Walter?

At her first day at school, Scout has to deal with a new teacher, Miss Caroline. Unfortunately, Miss Caroline is an inexperienced teacher who seems hopelessly out of her depth. She also doesn't seem to know much about the town and its residents. Her ignorance is there for all to see in her behavior towards Walter Cunningham. Walter can't afford to buy lunch, so Miss Caroline gives him a quarter, telling him he can pay her back the next day. But the Cunninghams, though dirt poor, are still intensely proud and won't accept charity from anyone, so there's no way that Walter will take Miss Caroline up on her well-meaning, but futile gesture.
Scout knows this. Atticus often does legal work for the Cunninghams. But as they can't afford to pay in him in cash for his services, they pay him in kind with goods such as hickory nuts and turnip greens. Through her father, Scout has learned the value of empathy, of putting herself in other people's shoes. So she knows that Walter will be made to feel like a charity case by Miss Caroline's offer to loan him his lunch money. She helpfully points out Walter's situation to Miss Caroline, but the inexperienced, deeply frustrated teacher sees Scout's intervention as nothing more than pure impertinence and so raps her across the hand with a ruler.