In chapter 3, Jem invites Walter Cunningham Jr. over for dinner and Scout is disgusted to witness him pour syrup all over his meat and vegetables. Since Scout is a naive, impulsive little girl, she rudely comments on Walter Jr.'s abnormal eating habits in front of everyone at the table by asking, "what the sam hill he was doing." Walter Jr. immediately feels ashamed and embarrassed following Scout's comments and ducks his head. Scout continues to comment on Walter's odd eating habits even after Atticus shakes his head, which prompts Calpurnia to intervene by requesting her presence in the kitchen. Once Scout walks into the kitchen, Calpurnia chastises her for her rude behavior and tells Scout that Walter Jr. should be treated with respect because he is her company. Calpurnia also tells Scout

Don’t matter who they are, anybody sets foot in this house’s yo‘ comp’ny, and don’t you let me catch you remarkin’ on their ways like you was so high and mighty! Yo‘ folks might be better’n the Cunninghams but it don’t count for nothin’ the way you’re disgracin‘ ’em—if you can’t act fit to eat at the table you can just set here and eat in the kitchen!

Before leaving the kitchen, Calpurnia spanks Scout and sends her back to the dinner table. Overall, Scout gets into trouble at the dinner table for rudely commenting on Walter Jr.'s eating habits and embarrassing her guest.

You are probably referring to the scolding that Scout receives from Calpurnia in Chapter Three. On the first day of school (Chapter Two), Scout gets off "on the wrong foot" with her new teacher, Miss Caroline. She blames Walter Cunningham Jr., so she rubs his nose in the dirt at the beginning of lunch hour. Jem breaks it up and invites Walter back to the Finch home for a home-cooked meal. Atticus is happy to see young Walter and treats him like a welcome guest. But Scout, still angry at him, makes fun of him for pouring syrup over all of his food. Calpurnia escorts Scout from the kitchen and gives her a good lecture. Afterward, Scout demands that Atticus fire Calpurnia, but he refuses.