This is one of the first evaluations of a "growth-mindset" intervention at scale in a developing country. I randomly assigned 202 public secondary schools in Salta, Argentina, to a treatment group in which Grade 12 students were asked to read about the malleability of intelligence, write a letter to a classmate, and post their letters in their classroom, or to a control group. The intervention was implemented as intended. Yet, I find no evidence that it affected students' propensity to find tasks less intimidating, school climate, school performance, achievement, or post-secondary plans. I rule out small effects and find little evidence of heterogeneity. This study suggests that the intervention may be more challenging to replicate and scale than anticipated.