Following approximately 1,800 Finnish children, this longitudinal study examined the associations between students’ perceived social support (teacher-student relationships, family support, and peer support), behavioral engagement, cynicism towards school, and academic achievement during the transition from primary to lower secondary school. After controlling for parental educational level and cynicism before the school transition, the results showed that the more students experienced pre-transition peer support, the less they reported post-transition cynicism. Furthermore, pre-transition peer support contributed to students’ increased academic achievement and behavioral engagement after the transition through decreased cynicism. The findings indicated that peer support functions as a protective factor against cynicism as students move through the critical school transition phase and positively contributes to students’ behavioral engagement and academic achievement.