Changing pedagogical practices is a promising, cost-effective avenue for improving education in developing countries, especially when done without changing current inputs such as teachers and instruction time. This article presents the results of a randomized evaluation of a program that aimed at changing the pedagogical approach used to teach the existing national mathematics curriculum. The program provides tools to regular preschool teachers to use an inquiry- and problem-based learning approach to tailor instruction to preschoolers in Peru. The results show an improvement of overall mathematics outcomes, which persist for some content areas even one year after the program ended. In contrast to results from previous research that suggest mathematics programs are biased along gender and socioeconomic lines, there is no evidence of differential effects by gender, language spoken at home, or proxies for socioeconomic status. Results also imply persistent stronger impacts on students whose teachers have university degrees.