Previous studies have reported that adolescents were at higher risk for mental health disorders during and after the COVID-19 pandemic due to the characteristics of their developmental period and to the impact of social and mobility restrictions on their daily routines. In response to these reports, we designed, implemented, and evaluated the impact of a brief positive psychological intervention in a high school setting to increase adolescents' psychological well-being during the COVID-19 pandemic. A pre-post experimental study was conducted to compare the levels of anxiety, academic motivation, and performance changes in an experimental and control group. A final sample of 148 students from two Spanish high schools of the Catalan region participated in the study (M[subscript age] = 12.45; 48% female). Results showed that participants in the experimental group reported a reduction in anxiety and an increase in global motivation levels compared to the control group. Regression analyses also revealed that the intervention was a significant predictor of reduced anxiety, increased motivation, and fewer failed subjects, while high anxiety predicted decreased performance and reduced motivation. These findings suggest that brief positive psychological interventions implemented in high school classrooms could be a cost-effective and systemic preventive measure for increasing adolescents' psychological well-being and strengthening their general mental health both during and after periods of pandemic restrictions.