Background: Small schools have been promoted as an educational reform that is capable of improving student outcomes. However, a survey of the research on small schools indicates that much of the movement for decreasing school size is based primarily on correlational methods that do not control for selection effects in the data. In addition, several recent studies have suggested that smaller schools may be able to increase student attendance and graduation rates but that these gains might not necessarily translate into gains in student achievement. Purpose: This study investigates the potential effect of attending smaller schools on student mathematics achievement using propensity score matching techniques. Research Design: Data in the study are from the Educational Longitudinal Study of 2002 and represent over 12,000 high school students. Observed student responses from 10th and 12th grade are used in the analyses. Data Collection and Analysis: An estimate of the potential effect of attending smaller schools is determined by matching students in the largest schools to smaller schools of four different sizes using propensity score matching techniques. These methods are used to attempt to account for selection effects present in these data and to approximate what the effect of attending a smaller school would be in each case. Results: Results from the study suggest that simply switching students to smaller school environments does not necessarily raise the mathematics achievement of students in the largest schools. Further analysis indicated that there did not appear to be an optimal range of school sizes that would provide maximum levels of student mathematics achievement. Conclusion: This study suggests that creating smaller schools might not be the best mechanism to raise student achievement. It is suggested that policy makers make careful deliberations before deciding to invest in small schools as an educational reform, because it is hard to establish when they will or will not be successful. Further research is needed into what makes some small schools effective.