September, 2015* This report presents quantitative and qualitative evidence on the expansion of the New York City Department of Education’s School of One middle school mathematics program. The School of One program began pilot testing in the summer of 2009 and was first used as a full-time mathematics program by three NYC middle schools in the school year 2010-11. 1 An expansion of School of One into four additional middle schools, funded by a federal “i3” grant, occurred over the school years 2012-13 and 2013-14. As part of the grant requirements, an evaluation of program impacts was performed by the authors of this report, who are independent researchers. This evaluation is based on a randomized control trial (RCT) methodology, where the School of One “treatment” was assigned randomly among a set of candidate schools. School of One is therefore being compared to traditional mathematics instruction programs in the remaining “control” schools.