Objectives: There is a growing interest in mindfulness-based programs. Yet, research in the area is limited, and little is known about the factors that moderate the effects of these programs. The two-fold aim of this study was (1) to examine the effects of a mindfulness-based program on dispositional mindfulness, inattention and emotional lability, handwriting fluency, spelling accuracy, and composing quality, as well as school achievement; and (2) to evaluate the moderating role of lesson absences, intervention-related knowledge, and social validity. Method: Using a quasi-experimental design, 257 fourth graders were assigned to an experimental group receiving a mindfulness-based program (n = 130) or an active control group receiving a health-based program (n = 127). Both programs were implemented in the classroom for 8 weekly units, which included two 30-min sessions delivered by psychologists, followed by three 5-min sessions delivered by teachers. All children were evaluated before and after the programs. Results: Compared to the control condition, the mindfulness-based program resulted in higher levels of internal and external awareness, and decentering and nonreactivity, as well as better composing quality and mathematics grades. Lesson absences, intervention-related knowledge, and social validity did not moderate the effects of the mindfulness-based program. Conclusions: These findings support the integration of mindfulness practices in primary school as a means to improve children’s academic-related skills and ability to be mindful.

Objectives: Mindfulness-based programs (MBPs) seem to be a popular way to develop pre-adolescents’ regulation abilities, psychological health, and life satisfaction, especially in difficult times. However, research into the effects of MBPs and factors influencing their effectiveness is still scarce and mixed. In the interest of understanding how MBPs can effectively be used to enhance pre-adolescents’ regulation abilities, psychological health, and life satisfaction, this study aimed (a) to analyze and compare the effects of two 16-week-MBPs with different implementation dosages and (b) to evaluate the moderating role of participants characteristics on MBP effects. Method: During the COVID pandemic, we conducted this quasi-experimental study, in which we compared three groups of Portuguese sixth graders (n = 105): daily MBP group (one long plus four short lessons per week), weekly MBP group (one long lesson per week), and a control group receiving socioemotional instruction. In particular, we examined MBP effects on attentional control, emotion regulation, psychological distress, and life satisfaction. Also, we evaluated the moderating role of participants’ gender, age, socioeconomic status, and baseline status on these effects. Results: In comparison to the other groups, the daily MBP showed lower expressive suppression and stress symptoms as well as higher life satisfaction. Moreover, both MBP groups reported greater cognitive reappraisal than the control group. While the effects of MBPs on emotion regulation were found to be moderated by gender, age, and baseline expressive suppression, no additional moderating effects were observed. Conclusions: These findings support the perspective that MBPs can serve as a broad preventive strategy, effectively promoting pre-adolescents’ psychological health and life satisfaction during challenging times. Preregistration: This study is not preregistered.