What impact can after-school programs have on the educational achievement of high school students? A new study of Chicago's After School Matters (ASM) program -- which offers paid internships in the arts, technology, sports, and communications to teenagers in some of the city's most underserved schools -- finds a relationship between participating in after-school activities and higher class attendance, lower course failures and higher graduation rates. Even after taking into account student demographic characteristics and prior attendance records, students who participate in ASM miss fewer days of school than their classmates. Similarly, students who participated at the highest levels in the after-school program tended to fail fewer core academic courses (English, Math, Science, and Social Studies). Furthermore, over the course of their time in high school, students who were enrolled in ASM for three or more semesters and those who participated at the highest levels had higher rates of graduation and lower dropout rates than similar students who did not participate in the program. The findings in this report highlight the importance of further research into what leads students to participate in after-school programs and the factors that lead to higher engagement and retention once they are enrolled. A better understanding is crucial for improving enrollment in after-school programs such as ASM. Moreover, accounting for student factors that lead to a greater engagement in the program will lead to a clearer understanding of ASM's contribution to the positive outcomes -- independent of hard-to-observe student characteristics such as enthusiasm or dedication.