Background: Social withdrawal is known to precede and contribute to the development of internalization problems in shy children. This study examined the efficacy of a selective primary school-based psychosocial intervention for withdrawn children. Method: The sample comprised 82 Primary 4 children aged between 7 and 8 years from 7 schools across Northern Ireland. There were 31 children in the comparison group (50.6% male) and 57 children received the intervention (41.7% male). A 2 x 2 mixed-model design was used: group (intervention group vs. comparison group) x 3 time points (pre- vs. 10 postintervention vs. 12-week follow-up) with repeated measures on the time factor. Teachers completed the SDQ at the three time points to assess participants’ socio-emotional health status. Results: Pyramid participants showed greater reductions than the comparison group on the Emotional Symptom and Peer Problem factors following the intervention and their scores did not return to baseline levels at follow-up. Pyramid attendance accounted for approximately 12% and 9% of the variance in the Emotional Symptom and Peer Problems decreases respectively. Conclusions: Pyramid Plus helped to alleviate internalization problems in participants and initially had a positive impact on social re-integration. The incorporation of cognitive restructuring techniques may be a necessary intervention modification to affect participants’ emotion regulation strategies.