This study tested whether elementary school pupils' language proficiency, and its development from grade 4 to 6, is affected by ethnic and socioeconomic school class composition. On average, pupils in school classes with high concentrations of ethnic minorities performed worse than pupils in classes with low concentrations of ethnic minorities. However, when percentage of employed mothers and degree of variation in the fathers' incomes were taken into account, the initial negative effect of ethnic minority concentration on language proficiency in grade 4 fell to a non-significant level. In addition, when in grade 6 the degree of variation in the fathers' incomes was taken into account, the initial negative effect of the percentage of ethnic minority children was reduced substantially. It seems that pupils' lower language proficiency is related more to the variation of the parental incomes in a classroom than to the presence of high proportions of pupils from ethnic minorities.