Self-regulated learning (SRL) is a crucial competence in our rapidly changing society, yet its systematic promotion in the classroom remains limited. Addressing this gap, this study reports on a teacher-led intervention to promote SRL within reading tasks among 5th and 6th grade students. Although some interventions have been implemented to promote SRL, little attention has been paid to promoting SRL in primary schools. Building on a previous SRL intervention, the current study added cooperative learning among students and parental involvement, two aspects that are assumed to add value when fostering SRL. A randomized controlled field trial was designed to evaluate the effect of an intervention using pre-test, post-test and follow-up measures. A total of 757 students from 40 classes participated in the study. The data were analyzed using a multilevel approach. This study revealed no significant difference in SRL or reading comprehension outcomes in the post-test and the follow-up test between students of the experimental and control group. Further investigations showed that several aspects of treatment integrity had a significant impact on SRL outcome. This study encourages future SRL and reading intervention studies to assess and analyze the multiple aspects of treatment integrity.