Randomized trials are a common way to provide rigorous evidence on the impacts of education programs. This article discusses the trade-offs associated with study designs that involve random assignment of students within schools and describes the experience from one such study of Teach for America (TFA). The TFA experiment faced challenges with recruitment, randomization of students, and analysis. The solutions to those challenges may be instructive for experimenters who wish to study future interventions at the student or classroom level. The article concludes that within-school random assignment studies such as the TFA evaluation are challenging but, under the right conditions, are also feasible and potentially very rewarding in terms of generating useful evidence for policy.