Academic stress is linked to adolescent distress and perfectionism during the final years at school, with girls being at greater risk. The onset of the COVID-19 pandemic was an additional stressor that impacted student learning on a global scale. The present study examines the effectiveness of an intervention targeting Psychological Capital (PsyCap), comprising hope, efficacy, resilience, and optimism (HERO) to increase these HERO resources and assess its impact on mental health symptoms and subjective wellbeing outcomes among a cohort of Year 12 students (n = 82, Mage = 17.09, SD = 0.28, 99% identifying as female) from a girls school during the first year of the pandemic. Primary outcomes of anxiety symptoms, depressive symptoms, and flourishing and secondary outcomes of HERO variables and perfectionism were examined. There were no significant changes in primary outcomes. Significant changes in efficacy, optimism, omnibus PsyCap (HERO combined) and perfectionism were found at post-intervention. Findings indicate the intervention targeting HERO constructs may be promising for developing HERO capabilities in youth and reducing common areas of concern for students (e.g., perfectionism). Future research directions addressing limitations are discussed.