2.1. Trial design and study participants
The trial was a randomized controlled trial with two parallel groups (experimental and control) and data measurements at two time points (baseline and after 1 year). Cluster sampling method was adopted in this study. Among the five schools that could be contacted, one school was selected as the sample population of this study. The number of second-grade students in this school had met the required sample size. Complete data were obtained from 225 students at baseline. Students from 6 classes were randomly divided into the experimental group and the control group. There were 114 students (62 boys and 52 girls) in the experimental group and 111 students (59 boys and 52 girls) in the control group. One year later, complete outcome data were collected for 113 students (61 boys and 52 girls) in the experimental group and 96 (50 boys and 46 girls) in the control group. The loss to follow-up rate was 0.88% in the experimental group and 13.51% in the control group, respectively. During 1 year of follow-up, 13 participants in the control group asked to leave the study. When the outcome data was collected, one participant in the experimental group and two participants in the control group were absent. The effective sample size was 209 students, as shown in Flowchart of the study.