Although the research methodology literature includes empirical benchmarks for effect sizes and intraclass correlations to help researchers determine adequate sample sizes through power analysis, it does not include similar benchmarks that would assist proper planning for attrition. To help fill this void, this paper describes how researchers can incorporate student attrition in power analyses and provides empirical benchmarks for the amount of attrition one might expect when conducting a school-based study that follows students over multiple years. The paper incorporates parameters for student attrition in common minimum detectable effect size calculations, presents attrition benchmarks based on student mobility rates in nationally representative longitudinal surveys, and presents benchmarks based on a synthesis of published evaluation studies. The paper includes a demonstration of how researchers can use the attrition benchmarks to take student attrition into account in a power analysis.