This longitudinal study applied latent change score (LCS) modeling to examine individual changes in students' (N = 1205) academic engagement (behavioral and emotional), social competencies (relationship skills and social awareness), and classroom relationships (emotional support from teachers and collaborative peer relations). Average changes during the first year of lower secondary school were investigated, and an LCS model specifying that individual changes in social competencies are related to individual changes in academic engagement partially via individual changes in classroom relationships was tested. The results indicated an average decline for all variables, particularly emotional engagement. Changes in social competencies were associated with changes in classroom relationships and indirectly with changes in academic engagement via changes in emotional support from teachers. A direct association was found between changes in social awareness and behavioral engagement. The findings reflect that the promotion of social competencies in lower secondary school may foster positive classroom relationships and academic engagement, mainly via perceived social awareness for behavioral engagement or via emotional support from teachers for both dimensions of academic engagement.