Research in social psychology and education proposes that adopting a growth mindset of intelligence is an important mediator for the well-being and performance of students at school. As a consequence, wise interventions have been developed to target student mindsets and change their beliefs about how much their intelligence can grow with training and experience. However, the efficacy of mindset interventions is highly debated, as effect sizes vary widely across studies. Here, we hypothesized that the study environment and, in particular, the teacher's mindset about intelligence is an important moderator of mindset intervention efficacy. We tested this hypothesis by randomly assigning six middle schools from underprivileged neighborhoods in the Paris area in France to a no intervention condition, a condition with mindset interventions delivered only to the students, and a condition with mindset interventions for teachers and students. The results show that the combined teacher and student mindset intervention condition was the most efficient for increasing the student's growth mindset. This finding suggests that a short and easy-to-implement mindset intervention for teachers can help students develop a growth mindset.