Physical activity (PA) provides multiple benefits for cognition, physiological and mental health; however, evidence in healthy human populations, particularly adolescents, remains limited. In a global context characterized by increasing youth sedentary behavior and prolonged screen time, it is essential to assess their impact in order to design effective educational strategies. First, we examined the role of physical activity as a modulator of cognitive processes such as memory, creativity, and imagination, as well as the temporal dynamics of its effects on these processes. We observed cognitive improvements associated with physical activity only when it occurred immediately before the tests, whereas no such benefit was evident when the activity was separated by a one-hour interval. In addition, we evaluated the impact of screen time on different mental health parameters, specifically anxiety and depression. The results showed that longer screen time was associated with increased symptoms of mental health impairment. This effect was present in both males and females, but was more pronounced in the latter. Finally, we found that students with higher levels of screen time exhibited poorer performance on creativity tests compared to those with lower levels of exposure. Taken together, these findings provide evidence that both physical activity and screen time modulate, in different ways, cognitive processes such as memory, creativity, and imagination.