This paper reports the findings of an investigation of the effects of school organization and ability grouping on students' academic achievements in Great Britain. Data for the study came from the National Child Development Study (NCDS) conducted by the National Children's Bureau (NCB) of London. The NCDS surveyed virtually every child born in England, Scotland, and Wales during the week of March 3-9, 1958, and the present study included data from follow-up studies conducted when the cohort was 7, 11, and 16 years old. Types of schools attended by the cohort were: (1) comprehensive; (2) grammar; (3) secondary modern schools in the state sector, and (4) private schools. Some, but not all, of the schools practiced ability grouping. Measures included as control variables were social background, parent influences, and school influences. Results of the study showed that separation of students into ability groups had an effect on achievement test performance in both reading and mathematics. The four types of schools received students whose average earlier test performance varied systematically in ways that were consistent with the social definitions of the schools. The several types of ability groups included students whose earlier performance suggested they had the designated levels of ability. (LMO)