This study examined the factors associated with triage competency among school nurses in South Korea. Using a convenience sampling method, 386 school nurses employed in elementary, middle, or high schools completed a cross-sectional survey that included a modified version of the Triage Competency Scale for emergency room nurses. Information regarding experience working in schools and hospitals, education level, school types, age, emergency nursing care certifications, school locations, and serious emergency experience at school was collected. Analyses were performed using SPSS version 25.0, independent t-tests, analyses of variance, Spearman’s correlation, and ordinal logistic regression. Triage competency was higher for school nurses who were employed in metropolitan regions (odds ratio [OR] = 1.63, p = 0.017) and had serious emergency experience (OR = 1.76, p = 0.008). As the participants’ experience at schools or hospitals increased by one year, their triage competency score increased by 2% (OR = 1.02, p = 0.037) and 14% (OR = 1.14, p < 0.001), respectively. These findings could be used to develop policies and educational programs that promote school nurses’ triage competency. Further, they suggest the importance of establishing an organizational support system to develop guidelines and a feedback system to improve school nurses’ triage competency.