This article examines two critical questions related to equality of educational opportunity. First, does the academic advantage that was observed in Catholic high schools more than two decades ago continue to hold for contemporary students in Catholic middle schools? Second, how closely do different school sectors adhere to the common school ideal? Answers to these questions are central to efforts to reduce educational inequalities. The study reported here relies on data from sixth- and eighth-grade students in the Chicago School Study, a longitudinal survey of Chicago public schools, and the Chicago Catholic School Study, a survey of all the Catholic schools in the Archdiocese of Chicago. These school systems are the third largest public and largest Catholic school systems in the United States. The results show no consistent sector differences in achievement gains in sixth and eighth grades in reading and mathematics. Moreover, background characteristics vary by school sector, and neither sector eliminates all background effects on achievement. Compared to the Chicago public schools, Chicago Catholic schools reduce the effects of students' race and socioeconomic status on reading achievement but exacerbate the effects of race on mathematics achievement. (Contains 2 tables, 2 figures and 9 notes.)