Physical Activity, Sleep Quality, Physical Fitness, and Academic Performance as Factors Associated with Health-Related Quality of Life in Primary and Secondary School Students from the Canary Islands

Background/Objectives: Health-related quality of life is a key indicator of well-being during childhood and adolescence. This study analysed its association with lifestyle-related factors, body composition, physical fitness, academic performance, and sociodemographic variables. Methods: A cross-sectional study was conducted with 949 schoolchildren from the Canary Islands, Spain, with a mean age of 11.8 ± 1.78 years. Health-related quality of life, moderate-to-vigorous physical activity, and sleep problems were assessed using questionnaires, while physical fitness and body composition were measured using field-based tests and anthropometric assessments. Results: Schoolchildren in the highest health-related quality of life tertile showed higher moderate-to-vigorous physical activity, fewer sleep problems, lower body mass index and waist circumference, better lower-body explosive strength, higher cardiorespiratory fitness, and better academic indicators. The multivariable model explained 27.7% of the variance in health-related quality of life. Sleep problems showed the strongest inverse association (β = −0.352; p < 0.001), whereas Primary Education (β = 0.256; p < 0.001), perceived academic performance (β = 0.124; p < 0.001), moderate-to-vigorous physical activity (β = 0.104; p = 0.001), lower-body explosive strength (β = 0.090; p = 0.045), and male gender (β = 0.094; p = 0.003) were positively associated with health-related quality of life. Conclusions: Schoolchildren’s health-related quality of life is closely related to sleep-related difficulties and, to a lesser extent, to movement behaviour, lower-body fitness, perceived academic competence, and sociodemographic factors. These results may inform integrated school-based strategies that combine sleep education, opportunities for daily movement, fitness development, and support for students’ perceived academic competence.