Why won't Jem allow Scout to brag about Atticus at school?
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.