Psychological capital (PsyCap) comprising the positive psychological resources of hope, efficacy, resilience, and optimism (HERO) has strong empirical associations with increased wellbeing and reduced mental health symptoms in adult samples. Emerging studies of PsyCap among school-age students have also shown preliminary, positive associations between PsyCap and student wellbeing. The present study is the first to examine PsyCap-HERO constructs and associations with both mental health symptoms and subjective wellbeing in school-aged children and adolescents (aged 9–14 years). A convenience sample of Australian school students (N = 456, Mage = 11.54, SD = 1.20, 47% female) completed an online survey during class time. Measures of hope, efficacy, resilience, optimism, flourishing, anxiety, and depression previously well-validated in school samples were used. Significant associations between each HERO construct and flourishing, anxiety and depression symptoms in the expected direction were found, and importantly, the combination of HERO constructs was shown to be a stronger predictor of increased levels of student flourishing, and decreased levels of anxiety and depression symptoms, than individual HERO constructs. Findings indicate that student PsyCap may be a promising area of further investigation for schools, policymakers, clinicians and researchers looking to identify positive psychological resources in youth that may buffer poor mental health and promote wellbeing.