<p>This study analyzed the effectiveness of the Positive Attitude Upper Middle School (PAUMS) Social Emotional Learning (SEL) program on social anxiety and social withdrawal by comparing groups of Portuguese adolescents who completed the program before and after the COVID-19 confinement. In addition, it examined the role of school climate during this period. Participants were 713 adolescents (48.5% girls) attending the 7th and 8th grades (<i>M</i>_age = 12.49, <i>S.D.</i> = 1.06). Four hundred eighty-three students participated in the intervention groups, whereas 230 students were part of the control groups. Assignment to the control and intervention groups occurred at the classroom level. Growth Curve Analyses using three measurement points were employed to analyze self-reports, where ANOVAs were used to analyze teacher reports, because only two assessments were available. Although both groups showed positive results, the groups that completed the intervention after the confinement reported a steeper decline in social anxiety and social withdrawal than those who completed it before. Higher levels of school liking were associated with a steeper decline in social anxiety. These results highlight the importance of the preventive role of an SEL program over its promotive component during a critical event.</p>