Final answer:
One characteristic noted by Klein in school shootings by teenage boys is the cultural reinforcement of masculinity, excusing minor aggression as natural male behavior.
Step-by-step explanation:
According to Klein and others, one characteristic that stands out in school shootings perpetrated by teenage boys is the cultural reinforcement of masculinity through a permissive attitude towards minor acts of deviance among boys. This behavior is often excused with the phrase 'boys will be boys', suggesting such aggression is unchangeable and a natural part of being male. The expectations of gender roles and socialization processes teach individuals to conform to these social norms, potentially contributing to the justification of aggressive behaviors later expressed in extreme forms like school shootings.