This protocol outlines a mixed-methods cluster randomized controlled trial (CRCT) in upper secondary schools that will investigate how video-based online coaching can support teachers’ competences in teacher-student interactions (TSIs) and thereby improve student engagement and learning. The intervention will take place over a school year with at least 100 teachers and 2,000 of their students. During the implementation and one-year follow-up, we will conduct an extensive implementation and process evaluation (IPE) to understand the intervention's effects on TSI and student engagement and learning outcomes. This protocol outlines the background, design, intervention, and primary and secondary outcome measures as well as moderators.