Background: School-based mentoring provided by teachers contributes to the ultimate goal of education: all-round student development and a good mutual coexistence. Aims: This study investigates secondary school students' perceptions of a school climate and school-based mentoring provided by teachers; it also examines the relationship between the school climate and peer aggression, and the moderating effect that teacher-led school mentoring could have on this relationship. Methodology: A total of 441 students between 12 and 17 years old (M = 13.99, SD = 1.33) were surveyed in Cantabria (Spain); the high schools were selected by means of a random sampling of two-stage clusters. Findings: The results demonstrated a direct negative relationship between a school climate and school-based mentoring with respect to peer aggression victimization. Likewise, mentoring showed a moderating effect on the relationship between a school climate and peer aggression victimization. Further implications: These findings highlight the importance of the school climate in preventing peer aggression and the central role that school-based mentoring plays in the relationship between a school climate and peer aggression victimization.