Few interventions reduce inequality in reading achievement, let alone higher-order thinking skills, among adolescents. We study policy debate--an extracurricular activity focused on improving middle and high schoolers' critical thinking, argumentation, and policy analysis skills--in Boston schools serving large concentrations of economically disadvantaged students of color. Student fixed effects estimates show debate had positive impacts on English Language Arts (ELA) test scores of 0.13 SD, equivalent to 68% of a full year of average ninth-grade learning. Gains were concentrated on analytical more than rote subskills. We find no harm to math, attendance, or disciplinary records, and evidence of positive effects on high school graduation and postsecondary enrollment. Impacts were largest among students who were lowest achieving prior to joining debate.