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 (Mage = 4 years, 9 months, SDage = 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.