Calpurnia doesn't really tell Scout anything about her fight at school. However, when Jem and Scout bring Walter Cunningham to eat lunch with them, Scout ridicules Walter for pouring syrup over his entire meal. This is what Calpurnia scolds Scout for. Calpurnia tells Scout that she is to treat all of her guests with respect. She tells Scout that it isn't her place to tell her guests how to behave.
This is a defining moment for Scout even though she becomes extremely angry at Calpurnia for it. Calpurnia has taught her that economic class does not define how a person is to be treated. This lesson goes along with how Atticus has raised his children, which shows the unity in childrearing between Atticus and Calpurnia.