Over a decade of research has demonstrated the positive effects of extended school day programs on various elementary and middle school student outcomes, both in the short and long term. The efficacy of extended school day programs in promoting academic outcomes among high school students is less well understood. This study contributes to the existing literature by examining school attendance, credit attainment, and standardized reading and math scores in a group of students at risk of academic failure who participated in extended school day programming. The study compared their outcomes to those of a group of demographically similar students who did not participate in the program. The extended school day program is provided within a full-service Schools Uniting Neighborhoods (SUN) Community School (CS) in the Portland, Oregon metropolitan area. Results suggest an advantage for SUN students in terms of better school attendance and earning credits toward graduation, but not in terms of standardized test scores. Implications for future research and extended school day policy are discussed. (Contains 2 figures and 2 tables.)