This article seeks to examine how laboratory and practical work mediates the effects of school provision and teaching style, via students' opportunity to learn the subject matter, on students' physics achievement. As a result of careful composition of variable constructs, a causal model of classroom physics teaching at the sixth form (grade 12) level in Hong Kong was explicated with high explanatory power and parsimonious structural relationships. This study drew its data from the second IEA International Science Study. Replications of the analyses across school systems and grade levels will be valuable for comparing instructional effectiveness and locating exemplary teaching practices. 1992 British Educational Research Association