Results: Girls reported higher disability than boys (p = 0.01), and lower QoL in the domains of physical (p < 0.001) and emotional functioning (p < 0.01), psychosocial health (p = 0.02) and physical health summary score (p < 0.001), and on the total PedsQL score (p < 0.01). School backpack weight was similar in both genders (p = 0.61) and in participants with and without LBP (p = 0.15). After adjustments, participants with LBP reported lower physical functioning (p < 0.01), influencing lower physical health summary score (p < 0.01).