We kindly thank the students, school staff, and data collectors who made this study possible. This study was supported by the Natural Science Foundation of Hunan Province ( 2019JJ40118 ), the Foundation of Hunan Provincial Education Department ( 18A113 ), the Foundation of Hunan Provincial Social Science Achievement (XSP19YBZ081) to XHY. The funding agents had no role in study design, data collection, analysis and writing.