Schools with very few and relatively low-performing marginalized students may be most likely to trigger social identity threats (including stereotype threats) that contribute to racial disparities. We test this hypothesis by assessing variation in the benefits of a self-affirmation intervention designed to counteract social identity threat in a randomized trial in all 11 middle schools in Madison, Wisconsin. We find that school context moderates the benefits of self-affirmation for black and Hispanic students' grades, with partial support among standardized achievement outcomes. Self-affirmation reduced the very large racial achievement gap in overall grade point average by 12.5 percent in high-threat school contexts and had no effect in low-threat contexts. These self-affirmation activities have the potential to help close some of the largest racial/ethnic achievement gaps, though only in specific school contexts.