The effects of academic class composition on academic progress in elementary school for students with different levels of initial academic abilities

This study aimed to estimate the effects of two indicators of academic class composition (i.e., class heterogeneity and average class achievement) on students' progress in mathematics and reading during the first grade in Russia. Using two-wave longitudinal data from the international Performance Indicators in Primary Schools (iPIPS), we examined a representative sample of 5118 first-graders (50% were girls, mean age was 7.3 years) to estimate the variability of these effects among students with different levels of initial academic ability. Multilevel regression analysis was performed, and the effects of average class achievements and class heterogeneity were significant and positive for reading but not significant for math. According to our analysis, the effect of average class achievement on reading was stronger for students with high initial academic ability, whereas the effect of class heterogeneity was higher for students with low initial academic ability. For math achievement, the effect of average class achievement was significant and positive only for students with high initial academic ability.