We revisit Harry L. Gracey's perspective of kindergarten as academic boot camp where, at school entry, children acquire the student role through a structured program of activities. We provide further insights into the crucial mechanisms of socialization that occur in U.S. kindergartens by examining the relationship between within-class ability grouping and attributes of children's psycho-social development that are critical for school success. Using data from the Early Childhood Longitudinal Study (ECLS-K) and incorporating stratified propensity scores into multi-level models, our analyses link within-class ability grouping to student behaviors in a manner that supports the concerns raised by ability grouping critics. Children in low ability groups show slower development in approaches to learning than comparable ungrouped children. Children in high ability groups show more accelerated development in all psycho-social attributes considered (approaches to learning, interpersonal skills, self control and externalizing problem behaviors) than comparable ungrouped children. These asymmetrical findings suggest that children placed in high and low ability groups are socialized into different versions of the student role.