- Schools and classes are important , although much of the variation in mathematics achievement is due to individual student characteristics.
- Results indicate that both the school and class/teacher level are relevant for the mathematics achievement of students. Especially the class composition in terms of the level of initial cognitive ability and SES and -to a lesser extent- the learning climate were important for explaining variance at the class and school levels.
- Results report important recruitment differences between schools and classes within schools. The group composition at the class level remained very important for the explanation of differences in mathematics achievement.
- The learning climate in the class, which correlates positively with group composition, explained an additional part of the variance.
- At the school level, the proportion of girls was positively related to mathematics achievement.
- Indications of differential effectiveness of classes and schools and of heteroscedasticity related to SES, average class SES, sex, and learning climate were found.
- Results indicate that ability grouping is often beneficial for talented students. But often harmful for the less able ones.