School bullying is a pervasive global issue with profound implications for students' well-being and academic performance. While extensive research has examined the role of bystanders--individuals who witness bullying--in mitigating such behavior, the specific influence of classroom climate on the emergence of positive bystander behavior, particularly through psychological mechanisms such as personal belief in a just world (PBJW) and empathy, remains underexplored. This study addresses the critical question of how classroom climate affects positive bystander behavior in instances of school bullying and seeks to identify the underlying psychological mechanisms at play. Utilizing a mixed-methods approach, data were collected from 670 students aged 11-13 in Chongzuo City, China. The findings reveal that a positive classroom climate significantly enhances positive bystander behavior, both directly and indirectly, through the mediating roles of PBJW--defined as the belief that the world is fundamentally fair--and empathy, characterized by the capacity to understand and share the feelings of others. Specifically, a supportive classroom environment fosters stronger just-world beliefs and greater empathy among students, thereby promoting proactive bystander intervention during bullying incidents. These insights underscore the critical importance of cultivating a nurturing and equitable classroom atmosphere, suggesting that such environments can effectively empower students to engage in positive bystander behavior, thereby reducing the incidence of bullying.