As students progress through the educational system they make frequent transitions, such as from grade to grade or middle to high school. The purpose of this study is to explore how institutional discontinuities between schools affect student achievement in mathematics and science and for which types of students. This paper seeks to identify specific discontinuities that aggravate the transitional experience, and to discover support structures that buffer the effect of the transition from middle to high school on the mathematics and science progress of students. Findings indicate that students from more stable and supportive home environments show less academic difficulty as they progress through the transition. Decreases in safety and the quality of the learning environment had significantly negative effects on achievement across the transition for both mathematics and science. A decrease in the degree to which teachers push students to achieve had a positive effect on student progress, and an increase in the level of autonomy granted to students and their parents to choose courses had a negative effect on student performance. There would appear to be a need for supplemental programming during the transition between middle and high school levels. Several policy interventions are suggested for consideration. Contains 48 references. (PVD)