IMPORTANCE: Schools have been suspended nationwide in 188 countries, and classes have shifted to home-based distance learning models to control the spread of the coronavirus disease 2019 (COVID-19) pandemic. Additional information is needed to determine mental health status among school-aged children and adolescents during this public health crisis and the risk factors associated with psychological distress during the pandemic. OBJECTIVE: To assess self-reported psychological distress among school-aged children and adolescents associated with the COVID-19 pandemic. DESIGN, SETTING, AND PARTICIPANTS: This cross-sectional study using data from a survey on the mental health of school-aged children and adolescents in Guangdong province, China, conducted by using a stratified cluster random sampling method between March 8 to 30, 2020. To estimate outcomes associated with location of districts, only data from students with internet protocol addresses and current addresses in Guangdong were included. Data were analyzed from April 5 to July 20, 2020. EXPOSURE: Home-based distance learning during the COVID-19 pandemic. MAIN OUTCOME AND MEASURES: The main outcome was self-reported psychological distress, measured using the total score on the 12-item General Health Questionnaire of 3 or greater. Multivariate logistic regression was used to analyze risk factors associated with mental health status. Odds ratios (ORs) were used to analyze the associations of factors with psychological distress. RESULTS: Among 1 310 600 students who completed the survey, 1 199 320 students (mean [SD] age, 12.04 [3.01] years; 619 144 [51.6%] boys) were included in the final analysis. A total of 126 355 students (10.5%) self-reported psychological distress. Compared with students in primary school, high school students had increased risk of psychological distress (OR, 1.19 [95% CI, 1.15-1.23]). Compared with students who wore a face mask frequently, students who never wore a face mask had increased risk of psychological distress (OR, 2.59 [95% CI, 2.41-2.79]). Additionally, students who spent less than 0.5 hours exercising had increased odds of self-reported psychological distress compared with students who spent more than 1 hour exercising (OR, 1.64 [95% CI, 1.61-1.67]). CONCLUSIONS AND RELEVANCE: These findings suggest that the prevalence of self-reported psychological distress among students during the COVID-19 pandemic was relatively high. Frequency of wearing a face mask and time spent exercising were factors associated with mental health. Therefore, it may be necessary for governments, schools, and families to pay attention to the mental health of school-aged children and adolescents during the COVID-19 pandemic and take corresponding countermeasures to reduce the impact of the COVID-19 pandemic on students' mental health.