In Killing Mr. Griffin, Duncan explores the susceptibility of peer pressure among young people struggling to find themselves. This theme is important as a trope of young adult fiction because issues of peer pressure are relatable for young readers.

The most prevalent example of this theme is Susan’s struggle with peer pressure. Susan is identified by her loneliness and her outsider status. She feels self-conscious about her awkward looks, and her aptitude for academics gives her a nerdy perception at school. She is lonely and friendless. These internal conflicts make Susan ripe for being taken advantage of. Betsy introduces the idea of bringing Susan into the plan to kidnap Mr. Griffin because Betsy knows that Susan has a crush on Dave. By dangling Dave as a potential boyfriend, Betsy is confident that Susan will succumb to peer pressure. Susan is taken off guard when Betsy, Jeff, Dave, and Mark invite her out to a picnic to convince Susan to participate in the kidnapping plan. Susan quickly agrees to the plan despite her reservations because she can’t pass up on the opportunity to grow closer to other people.