A re opposite gender friends in high school a distraction or do they promote better academic achievement? Students spend a large amount of time with their school friends (Fuligni and Stevenson 1995;Gager, Cooney, and Call 1999), yet we know little about how the composition of their school friendship networks affects them. Understanding friendship network gender composition speaks to the single-sex education debate to the extent that changes in class and school gender composition affect both the formation of and interactions with opposite gender school friends.The single-sex versus mixed gender education debate has received renewed attention in the wake of Title IX regulations that eased constraints on single-sex education within the US public school system (Jackson 2012). Single-sex classes and schools may make it easier for educators to address gender gaps in achievement (Fortin, Oreopoulos, and Phipps 2013;Bertrand and Pan 2013;Ku and Kwak 2013) and traits such as competitiveness (Gneezy, Niederle, and Rustichini 2003;Niederle and Vesterlund 2010) that have been shown to affect labor market outcomes (Buser, Niederle, and Oosterbeek 2012). Identifying how the gender composition of a * Department of Economics, Darla Moore School of Business, University of South Carolina, 1014 Greene St, Columbia, SC 29208 (e-mail: andrew.hill@moore.sc.edu). I wish to thank Nicole Fortin for providing many hours of advice and supervision, Thomas Lemieux and Craig Riddell for very helpful suggestions,