This study addresses the issue of how different school organizational patterns affect the academic learning of students of different backgrounds and abilities. Using data from the Pennsylvania Educational Quality Assessment (EQA) on approximately 8,000 sixth-grade students in elementary and middle schools, the study examines how instructional specialization, between-class ability grouping, within-class ability grouping, and grade span affect the achievement of students from low to high SES backgrounds. The study finds that elementary school settings benefit students from low social backgrounds, as does having instruction provided by a limited number of teachers. Between-class ability grouping shows benefits for high social background students in middle schools, and within-class ability grouping in elementary schools benefits low social background students in reading. (Author/PCB)