We investigate ability and developmental differentiation as well as their interaction in school achievement test data in a large sample of Luxembourgian students. The data was collected in the context of the Luxembourgian school monitoring program Épreuves Standardisées (ÉpStan). The students were tested in grade 5 and grade 7, allowing for longitudinal analyses of developmental differentiation and the interaction effect. A second sample was tested in grade 9. Students in grade 9 attended one of two different school tracks, allowing to investigate the effect of educational environment on the structure of school relevant abilities. Nonlinear factor analysis is used to estimate the different effects in both samples.