This study investigated the effects of classroom composition on achievement. The analysis focused on the assessment of classroom homogeneity with respect to the socioeconomic background and general intelligence of the students. In addition, the extent to which classroom composition affects the standards the teachers employ and how these standards can account for the effects of classroom composition were examined. Two different data sets were analyzed: the first, which contains information about a national sample of 212 elementary schools, was used to assess the effects of classroom composition; the second data set, which contained information about a sample of 51 schools from four regions in the East and South of the Netherlands between May 1986 and June 1988, was used primarily to test whether teacher goals can account for the effects of classroom composition. The results suggested that: (1) an increase in classroom homogeneity may lead to a somewhat higher level of average achievement in Dutch elementary education, but also to a considerably larger variation; (2) the achievements of the least talented students are likely to decrease; (3) although teacher goals were found to exert a considerable effect on individual achievement, they could not account for the observed classroom composition effects; and (4) the effect of the classroom average on individual achievement was similar in size to the effect of socioeconomic background. Contains 43 references. (AA)