Background: Happiness is a measure of school-age children’s well-being. Many factors have been identified as the source of school-age children’s happiness, among which school experiences are particularly significant. This study aimed to investigate school-age children’s happiness and school experiences in various contexts. Methods: In total, 3282 Thai school-age children (first- and sixth-grade students) across the country were randomly selected in 2019. The exploratory research design was used for quantitative and qualitative data collection. The selected samples were administered self-reported questionnaires, interviewed, and observed in their school settings for five days a week. The collected quantitative data were analyzed with mean scores, standard deviations (SD), and the t test, while qualitative data were analyzed using content analysis. Results: The mean score (±SD) of the first- and sixth-grade students were 2.290 (±0.32) and 2.220 (±0.30), respectively, which was interpreted as a moderate level of happiness. Factors such as grade levels, school types, food consumption, class hours, learning activities, and play activities had a statistically significant impact on school-age children’s happiness (P≤0.05). A qualitative examination of their routines and schedules on weekdays demonstrated that these children lost out on personal time and could not make decisions to spend time on the activities of their own interest and preference. Conclusion: School-age children are burdened with required schoolwork and barely have the time for any recreation and rest. Thus, children’s happiness is reduced with lower recess time.