The purpose of the study was to explore how fixed and modifiable family, activity, and school factors affect a student’s myopia risk and severity. We used national cross-sectional data from Taiwanese children in Grades 4–6. Bivariate and multivariate analyses, including logistic and ordinary least squares regression, examined factors related to children’s myopia status and severity. Age, parent myopia, and school district were associated with risk of myopia. One hour or more per day of near work (OR = 1.26) increased the odds of myopia. The same amount of time in outdoor activities (OR = 0.85) or moderate or vigorous physical activities (OR = 0.82) was associated with lower risk. Near work (β = 0.06), outdoor activity (β = −0.04), and outdoor recess (β = −0.03) predicted myopia severity. To promote healthy vision, nurses should advocate for and implement interventions that increase school children’s time outdoors and in physical activities and reduce their time on near work.