Asthma is a common disease in children, and obtaining basic knowledge and skills to manage asthma symptoms is critically important. This study examined the effectiveness of a theoretically based school nurse-led asthma intervention on asthma symptoms, self-management, peak flow meter usage, daily activities, and school absences in children 7-12 years old. A randomized controlled, two-group, repeated measures design was conducted in eight public schools. Data collection occurred at three time points. The treatment group had statistically significant differences in reported symptoms (p < 0.001), asthma control with a peak flow meter usage (p < 0.001), and daily activities (p < 0.001) at 6 weeks and at 12 weeks. Although difference in school absences was not statistically significant, the treatment group missed fewer school days than the control group. Self-management is a complex process, and school nurses can provide essential learning steps and continuity of care for school-age children living with asthma.