BACKGROUND Schools are frequent settings for childhood obesity prevention; however, intervention results are mixed. Classroom context may hold important clues to improving these interventions. METHODS We used mixed methods to examine classroom context during a curriculum intervention taught by trained instructors in fifth grade classrooms. We identified classrooms with high and low buy‐in using cluster analysis and compared intervention delivery and reception, student energy balance‐related behavior, and student perceptions about supports and barriers to energy balance. RESULTS Delivery and reception did not differ between the groups. Between‐group differences in unhealthy behaviors were smaller at posttest, due to improvement in low buy‐in classes. Although student perceptions of supports and barriers to energy balance were similar across groups, students in high buy‐in classes indicated food preferences as a support while students in low buy‐in classes indicated food preferences as a barrier. Neighborhood environment emerged as a universal barrier. CONCLUSIONS Trained instructors may improve intervention delivery and reception regardless of classroom context. Social norms could explain high levels of unhealthy behavior in low buy‐in classes at baseline; improvement at posttest suggests that the curriculum may have motivated change. All children need more strategies and supportive policies to overcome a challenging food environment.