The relationships between reasoning and school achievement were studied taking into account the multilevel nature (school- and class-levels) of the data. We gathered data from 51 classes at seven schools in metropolitan and Eastern Finland (N = 769, 395 males, 15-year-old students). To study scientific reasoning, we used a modified version of Science Reasoning Tasks, tapping control-of-variable schemata. Analyses were conducted by MLwiN2.10 multilevel modelling. The present results showed that the intra-class correlation coefficient (ICC) of schools for scientific reasoning is 7% and that the corresponding ICC of classes is 10%. Whereas the first finding confirms earlier PISA results, the second finding provides new insights into class variation within schools. In practice, class composition seems to be an efficient solution to meeting the differing needs of individual students.