Despite a vast international literature, previous research into compositional effects and peer effects in education has remarked on the lack of consensus within the field. To date, peer effects in education and students’ different opportunities to learn in school have been studied separately. In this thesis, it is argued that these perspectives need to be synthesized.

In the thesis, Frame Factor Theory is employed as a theory of peer effects in classroom instruction. According to the theory, one mechanism generating peer effects is the steering and limiting effect that class composition has on teachers’ instruction. It is argued that this perspective also needs to be synthesized with the Opportunity to Learn-perspective, to widen our understanding of why different students meet different instruction and thus are given different opportunities to learn. The theoretical perspectives and empirical models are evolved through three empirical studies included in the thesis. The theoretical development and the empirical results from each study is discussed in an integrative essay in the thesis.

The results indicate that class composition affects the presence of limitations on instruction and content coverage in instruction, both of which is related to students’ opportunities to learn and individual students’ achievement. Implications for future research and educational policy is discussed.