Background: Social relationships can serve as important risk or protective factors for child development in general, and academic adjustment in particular. Aims: This study investigated the role of teacher-child interactions in academic adjustment among preschool boys at risk of externalizing behaviour, using a randomized controlled trial study with Playing-2-gether (P2G), a 12-week indicated two-component intervention aimed at improving the affective quality of the teacher-child relationship and teacher behaviour management. Sample: In a sample of 175 preschool boys showing signs of externalizing behaviour (M[subscript age] = 4 years, 9 months, SD[subscript age] = 7 months) and their teachers, we investigated P2G effects on academic engagement as well as on language achievement. Methods: Academic engagement was rated by teachers at three occasions within one school year (T1 = pretest, T3 = post-test, and T2 = in-between intervention components). Language achievement was assessed by researchers at pre- and post-test, using a standardized test. Results: Cross-lagged path analyses revealed a direct intervention effect of P2G on academic engagement at Time 2. In addition, a significant indirect intervention effect was found on academic engagement at Time 3 through academic engagement at Time 2. Finally, academic engagement at Time 2 was found to predict language achievement at post-test. A marginally significant indirect intervention effect was found on language achievement at Time 3, through academic engagement at Time 2. Conclusions: This intervention study suggests that teacher-child interactions predict academic engagement over time, which in turn improves language achievement among preschool boys at risk of externalizing behaviour.