In an attempt to forge tighter social relations, small school reformers advocate school designs intended to create smaller, more trusting, and more collaborative settings. These efforts to enhance students' social capital in the form of social closure are ultimately tied to improving academic outcomes. Using data derived from ELS: 2002, this study employs propensity scores in the context of multilevel models to estimate the effects of a specific school-level variant of social closure, referred to as a norm-enforcing school, on students' mathematics achievement. Results estimate that attending a norm-enforcing school has no effect on 12th-grade mathematics achievement. This result questions the presumed benefits of social capital and its emphasis on norm-enforcement and social control. Policy implications are discussed in light of contemporary urban school reform initiatives that focus on reductions in school size.