In this article an instructional model of cooperative learning in secondary mathematics education was put to the test. Students worked in cooperative groups of four on real life mathematical problems based on Freudenthal’s adagium 'Mathematics for All' and his theory of ‘Realistic Mathematics Education’ (RME). The data were collected at three secondary schools, with a total of 482 pupils, 12 teachers and 22 classes. The curriculum was implemented in the second year of secondary education with a majority of pupils in the ages of 13 and 14 years. The research design was pretest-posttest. Differences in implementation were found, in particular, in the extent to which the Freudenthal model had been implemented with regard to schools, teachers and pupils. We investigate the effects of the curriculum by relating these differences in implementation to differences between students' learning results. Given the basic idea behind 'Mathematics for All', the research question about the differential effects of this curriculum is important. Which pupils benefit most from the implementation of the new curriculum and how can we explain the differences in learning results?