The differences between reading and mathematics achievement were studied for 10 high and 10 low socioeconomic status (SES) elementary schools in a midwest urban district. The schools representing the top and bottom quartiles from among 40 schools were classified as high or low SES schools by using a set of highly intercorrelated school level SES indicators. These indicators reflect the out-of-school environment and experiences of students that are outside of school control. The gap of almost one standard deviation in mean reading and mathematics achievement levels between these quartiles remained nearly constant as the students progressed from grade two through grade six, and the achievement gap was consistently a little larger for reading than for mathematics. Associated with the achievement gap was a student behavior gap reflected in attendance and suspension rates. The achievement and behavior gaps were highly correlated. Because of the interrelationships between the SES indicators and the percent of minority students the achievement and behavior gaps can not be attributed to the percent of minority students within the elementary schools. (Contains nine references.) (Author/SLD)