Abstract Background This study aimed to examine the impact of a web-based positive psychology program delivered universally to secondary school students during school closures caused by the COVID-19 pandemic in New South Wales, Australia. Methods Using a quasi-experimental design conducted in 2020, 438 students aged 12–15 years (73% male) from 4 secondary schools were invited to complete the ‘Bite Back Mental Fitness Challenge’. This web-based program consisted of 7 self-directed modules that targeted 5 key domains of positive psychology. Self-reported symptoms of anxiety and depression and help-seeking intentions for mental health were assessed at baseline prior to school closures (February to March 2020) and at post-test after the return to school (July to August 2020). At post-test, students also reported on their perceived changes in mental health and help-seeking behavior for mental health during the pandemic. Completion of the program modules was recorded. Results A total of 445 students consented and 336 (75.5%) completed both assessments. On average, participants completed 2.31 modules (SD: 2.38, range: 0 to 7). There was no change in symptoms of anxiety and depression or help-seeking intentions between baseline and post-test, with no significant effects for gender and history of mental illness. Students who were symptomatic for anxiety and depression at baseline reported lower symptoms at post-test, but this change was not significant. Ninety-seven students (27.5%) reported that their mental health had worsened during the pandemic, and a significant increase in anxiety and depressive symptoms was found in this subsample at post-test. Only 7.7% of students reported a change in their help-seeking behavior, with increased mental health support sought from the Internet, parents, and friends. Conclusions The universal delivery of a web-based positive psychology program during school closures did not appear to be associated with improved mental health symptoms; however, completion of the modules was low. Different effects may emerge when selectively delivered to students with mild or greater symptoms. The findings also suggest that broader measures of mental health and wellbeing, including perceived change, are key to the mental health surveillance of students during periods of remote learning.