School-based mindfulness programs (SBMPs) have gained global popularity. Yet, there is a need for more rigorous procedures to develop and assess them. This study aimed to address these limitations by examining the effectiveness of a Portuguese SBMP, called SER+ FELIZ(mente). The final sample included 190 third- and fourth graders: 99 in the SBMP group and 91 in a wait-list group. Effects were examined on attentional control, emotional regulation, and wellbeing in the short term (T2) and 6 months later (T3). We also tested the moderating role of age and gender. Using a multilevel modeling approach, results showed a clear benefit of SBMP on emotional wellbeing at T2, but not T3. Despite SBMP students surpassed wait-list students in attentional control, cognitive reappraisal, and social wellbeing at T2, these effects were due to a levelling off among SBMP students coupled with a decline among wait-list students from T1 to T2. These ef-fects were moderated by age at T3. While universal improvements were limited, our SBMP seemed to have acted as a shield against school stressors (i.e., likely, test anxiety at T2). Future research is, however, needed to further explore the value of SBMP to protect students’ mental health.