Young people face multiple challenges, including appearance dissatisfaction, academic stressors, anxiety and depression. These challenges may increase during the final year of high school and may have become further exacerbated by the COVID-19 pandemic. This study examines the preliminary effectiveness of a brief, uncontrolled school-based intervention aimed at enhancing Psychological Capital (PsyCap), consisting of positive resources of hope, self-efficacy, resilience and optimism (HERO), and reducing mental health symptoms among female year 12 students (n = 95, Mage = 16.78, SD = 0.45). Outcomes on measures of HERO and secondary outcomes of flourishing, appearance dissatisfaction, anxiety and depression were measured at pre and post-intervention. In the overall cohort, no significant changes were found on the HERO outcomes or flourishing post-intervention, while symptoms of depression, anxiety and appearance dissatisfaction decreased significantly. For students with higher baseline anxiety, optimism and anxiety symptoms improved significantly at post-intervention. However, students with lower baseline anxiety experienced significant decreases in self-efficacy and optimism, as well as a significant increase in anxiety symptoms post-intervention. Baseline depression levels did not impact intervention outcomes. The findings, although mixed and limited by the lack of control group, suggest that a brief intervention grounded in PsyCap theory may improve student wellbeing under challenging circumstances.