This paper evaluates the causal impact on academic achievement of contracting out public schools to private operators. The Colegios en Concesion (CEC) program is a large-scale initia-tive implemented in 2000 in Bogotá, Colombia, that contracted out all new public schools to reputed, not-for-profit private schools and universities. This program allowed CEC schools to op-erate outside the collective bargaining provisions of traditional public schools in return for higher accountability, including test-based accountability. To overcome potential selection bias of CEC school attendance, I exploit variation in distance from a student’s residence to the closest CEC institution as an instrument for CEC attendance. While distance may in theory be correlated with unobservable characteristics of students, I demonstrate using a variety of empirical strategies that this instrument is conditionally exogenous of unobserved determinants of academic achievement. The two-stage least squares estimates obtained indicate that CEC students score 0.6 and 0.2 stan-dard deviations higher in math and verbal tests, respectively, relative to public school students. I provide evidence that the estimated results are not driven by unintended strategic responses by CEC schools, such as excluding low-performing students from the pool of test-takers, di˙erential dropout rates, or via test specialization in the curriculum.