This study examines the impact of grade retention on behavioral outcomes under a comprehensive assessment-based student promotion policy in New York City. To isolate the causal effect of grade retention, we implement a fuzzy regression discontinuity (RD) design that exploits that grade retention is largely determined by whether a student scores below a cutoff on a standardized test score. We use data on students subject to the policy over a nine year span to examine impacts on attendance and disciplinary event outcomes. We do not find evidence of systematic effects of retention on behavioral outcomes in either direction. We do find sporadic non-sustained significant effects of retention on behavioral outcomes. When present, these isolated non-persistent effects tend to be beneficial when found for retained elementary school students and mixed for retained middle school students. [This paper was published in "Journal of Research on Educational Effectiveness" v11 n2 p192-216 2018 (EJ1175243).]