Among a sample of 2,071 Australian high school students nested within 188 science classrooms, the present study explored the role of science teachers' "load reduction instruction" (LRI; instruction that seeks to reduce cognitive load by appropriately balancing explicit instruction with guided autonomy) in student- and classroom-level science engagement and the role of engagement in student- and classroom-level science achievement. Using doubly latent multilevel structural equation modeling, results showed that, at the student- and classroom-level, student reports of their teacher's LRI was significantly and positively associated with self-reported engagement, and engagement was significantly and positively associated with achievement. Thus, (a) LRI was associated with greater individual student engagement, that in turn was associated with greater individual student achievement in science, and (b) beyond student-level effects, LRI was associated with greater classroom engagement that in turn was associated with greater classroom achievement. We also found that the association between LRI and achievement was mediated by engagement. Implications for educational practice in science are discussed.