The low cost private school (LCPS) sector is spreading rapidly around the world. In some Nairobi neighborhoods, more than half of children attend LCPS, despite the implementation of Free Primary Education providing free access to public education. Parents generally choose LCPS because they believe they are higher quality (Authors, under review), though there is little conclusive evidence of this. This paper usees gain scores—as opposed to cross-sectional measures—to compare the outcomes of students attending low cost private schools (LCPS) and public primary schools. We therefore discuss how these schools impact children’s achievement over time, using a longitudinal sample of 326 children. We argue that this approach is less biased than the cross-sectional approaches used in earlier work. We find that LCPS do not produce higher student growth over time than public schools. However, among the schools that were participating in an instructional improvement intervention, LCPS’s increased student performance more than public schools, particularly in English. We also present data comparing the teacher populations of LCPS and public schools, which helps to posit reasons for LCPS’ greater responsiveness to the intervention. These findings offer a cautionary note to the reliance on LCPS as a means of expanding quality education in low-resource settings.