In this study, we analyzed the participation of teachers and students during their co-construction of explanatory models for concepts in circuit electricity in two high school physics classes. While students in both teachers’ classes experienced comparable levels of impressive pre to post-instructional test gain differences over controls, analysis of class discussions showed that considerable differences existed between the two groups in the ratios of student-to-teacher contributions to the development of explanatory models. Applying a new cognitive framework for the analysis of classroom dialogue (Williams & Clement, 2015), teacher and student contributions at the non-formal reasoning level were coded into model construction process categories of: referring to observations (O), generating explanatory models to explain phenomena (G), evaluating models currently under discussion (E), and making modifications to these models (M). This analysis based on the OGEM modeling processes made it possible to categorize each teacher and student contribution and to describe the specifics of how the model co-construction process was shared in each classroom. Ratios of teacher to student contributions in each category differed markedly between the two teachers. We conclude that teachers may vary in their styles and degrees of participation in model co-construction processes and still produce similar gains in conceptual understanding. We hypothesize that what remains most important is their ability to foster students’ engagement in the four key processes of modeling.