Background: This study aimed to explore the effectiveness of a 12-week circuit training program in improving local muscular endurance in normal-weighted primary school students. Methods: The study involved a parallel-group randomized trial with 606 primary school boys assigned to an experimental or a control group. The participants underwent a 12-week circuit training program that involved multi-joint, total-body workouts using body weight, resistance bands, and medicine ball exercises. The study measured the participants’ local muscular endurance during sit-ups, dynamic trunk extensions on a Roman bench (DTE), and push-ups. Results: After adjusting for the baseline, the treatment–grade interaction was significant for sit-ups (F = 7.74, p < 0.001, η_p² = 0.04), DTE (F = 6.49, p < 0.001, η_p² = 0.03), and push-ups (F = 9.22, p < 0.001, η_p² = 0.05), where the experimental treatment was more beneficial than the control. The treatment effect seemed to vary depending on the individual’s baseline local muscle endurance capacity. As the baseline local muscular endurance values increased, the treatment and grade effects became less beneficial. Conclusion: A 12-week circuit training program involving body weight, resistance bands, and medicine ball exercises suits school-based programs and can improve local muscular endurance in normal-weighted primary school boys. The experimental treatment was more effective than the control, and the individual baseline muscular endurance should be considered when designing training programs.

The COVID-19 pandemic has impacted almost every aspect of life, especially daily physical activity and healthy eating habits but also mental health. Our study aimed to examine the relationship between the physical activity level, eating habits and mental health of Serbian adolescents during the COVID-19 pandemic. A total of 3506 students from the territory of the Republic of Serbia participated in this study. IPAQ-short version and HBSC-FFQ were used to assess physical activity level and eating habits, along with self-rated health. Moderate correlations were identified between physical activity, eating habits and mental health, along with average physical activity, very high life satisfaction (β = 0.177, p < 0.01) and very low emotional distress (β = −0.150, p < 0.01). A significant predictor of mental health was the frequency of breakfast on weekdays (β = 0.167, p < 0.01 for life satisfaction and β = −0.153, p < 0.01 for emotional distress), but not on weekends. Since the pandemic’s course is uncertain, the focus should be on maintaining good physical activity, nutrition and well-being.