Kindergarten readiness assessments are commonly used in schools nationwide. Prior work shows that the use of such assessments for class placement decisions has increased in recent years. This article uses the Early Childhood Longitudinal Study of 2011 to explore whether the use of readiness assessments for such purpose predicts differential sorting of students across classrooms by prior academic ability. Results from multilevel models as well as other sensitivity analyses suggest that the use of readiness assessments for classroom placements is predictive of slightly higher cross-class ability sorting, particularly in English/language arts. The implications for policy and practice are discussed.