Why won't Jem allow Scout to brag about Atticus at school?
In Chapter 10, Jem and Scout are astonished to discover that their seemingly boring father is an expert marksman after he shoots and kills a rabid dog in one shot. After Atticus shoots the dog, Jem is at a loss for words and Miss Maudie tells the children that Atticus earned the nickname Ol' One-Shot as a boy for his marksmanship abilities. Jem then wonders why Atticus never mentioned that he was the best shot in Maycomb, and Maudie essentially tells Jem that his father is too civilized and humble to brag about his God-given talent. Maudie also tells Jem,
"People in their right minds never take pride in their talents" (Lee, 102).
Later that day, Scout tells Jem that she cannot wait to brag to her friends about Atticus's marksmanship abilities, and Jem responds by telling her, "Don’t say anything about it, Scout" (Lee, 102). The reason Jem won't allow Scout to brag about Atticus's talent is because he wants to be a gentleman like his father and understands that having the ability to shoot beings is not something to take pride in. After listening to Maudie's explanation, Jem realizes that it is not decorous or honorable to brag about one's talents, and he realizes that Atticus does not take pleasure in killing living beings.
In Chapter 10 of Harper Lee's To Kill a Mockingbird, Scout learns for the first time that Atticus is a sharpshooter. Having previously thought that Atticus is feeble because of what Scout thinks is his old age, Scout is very impressed to learn that Atticus can do something worthy of boasting about. Yet, Jem tells her not to boast about it because he understands Atticus is ashamed of his ability to take life so easily.
Atticus displays his sharpshooting skills when he is forced to rescue the neighborhood from a rabid dog. By the end of the chapter, Scout comments to Jem that they would "really have something to talk about at school on Monday," but Jem warns her not to say anything about Atticus's shooting skills. Jem understands that Atticus gave up shooting as soon as he comprehended, as Miss Maudie phrases it, that "God had given him an unfair advantage over most living things." Since Jem understands how much Atticus values life, he also understands why Atticus gave up shooting and has kept quiet about his skills all this time. As Jem explains to Scout, "Atticus is a gentleman, just like me!," meaning one who shows respect for and values all of life.