Building on previous research showing how well ninth-grade student behaviors predict on-time high school graduation, this experimental study investigates the impact of a ninth-grade intervention on student attendance and course passing. The study, conducted in 41 geographically and demographically diverse high schools within a single state, evaluates the effects of placing a half-time staff member in high schools to implement the Early Warning Intervention (EWI) Team model designed to monitor ninth-grade early warning indicators and provide timely interventions. Analyses based on the pre-specified student outcomes of attendance rate and percentage of ninth-grade course credits earned indicated no statistically significant impact of the intervention. On secondary outcome variables, results indicated that students in treatment schools were significantly less likely than control school students to be chronically absent. The difference between treatment and control school students on dichotomous measures of course failure were not statistically significant. The widespread dissemination of research and best practices related to early warning systems and ninth-grade interventions likely accounted for low levels of contrast between treatment and control school practices and outcomes.