Drawing on data from "Prospects: The Congressionally Mandated Study of Educational Growth and Opportunity," this issue brief explores the magnitude of reading and mathematics differences in top and bottom schools. Four types of comparisons are made: (1) comparing the percentile ranks of the average student in the top and bottom quarters of schools at the end of first grade; (2) converting test score differences between students in the top and bottom quarters of schools into a widely used measure called effect size; (3) determining the number of years of schooling the bottom school students are behind the top school students at the end of first grade; and (4) showing how students in the bottom quarter rank on average at the end of first grade relative to students in the top schools and vice versa. In addition, the educational and economic backgrounds of the students attending the top and bottom quarters of schools in achievement are noted. Each of the four approaches show large differences in reading and mathematics achievement at the end of first grade between students in the top and bottom schools. Data on family educational and economic backgrounds also show great differences between the top school students and bottom school students. (HTH)