<p>Few large Social and Emotional Learning program trials have been conducted outside the USA. This dissemination study analyzed the effectiveness of the Positive Attitude Upper Middle program in Portugal. The sample was composed of 2231 middle school students (<i>M</i>_<i>age</i> = 12.63, <i>SD</i> = 1.09; 47.3% girls), 669 in control groups, and 1562 distributed between the two intervention groups (led by experienced or inexperienced implementers). Students and their teachers reported that students from both intervention groups displayed larger gains in self-control, social awareness, relationship skills and responsible decision-making. For self-control, boys benefited more than girls and experienced implementers led intervention group students to larger gains. Thus, the current study supported the programs’ effectiveness and highlighted the role of implementer experience.</p>