With an increasing number of immigrant children in many countries, questions of how to prepare them for further education become highly salient. Few studies have examined the effect of first-language instruction on children’s engagement in school and how that may later transfer into better majority-language outcomes. A randomized controlled trial in Denmark (n = 230) took an asset-based approach to students’ cultural and linguistic backgrounds. We found that first-language instruction of majority-language learners (average age 7.1 years) reduced their behavioural problems in school and increased their school satisfaction and their parents’ engagement. We saw no immediate effect on their spoken first-language skills, but one year after the intervention ended, their reading skills in the majority language were substantially improved. Half of this improvement could be explained by reduced behavioural problems. The results thereby indicate that an asset-based approach to students’ linguistic and cultural backgrounds can help to ensure that first-language instruction transfers into majority-language skills.