Expanded instructional time has become increasingly popular as a strategy to improve the academic outcomes of low-skilled students, particularly in the 9th grade. We evaluate the efficacy of a double-period algebra policy initiated in the Chicago Public Schools in 2003. This policy required all students with 8th-grade test scores below the national median to enroll in a support algebra course in addition to regular algebra in the 9th grade. Using regression discontinuity combined with interrupted time series designs, and instrumental variable models, we show the effects of the policy on students' grades, failure rates and test scores in 9th-grade algebra and 10th-grade geometry. Providing support courses improved algebra test scores for the target population but only modestly affected grades and failure rates. Students with very low initial abilities benefited less than students close to the national median. The policy also led schools to track algebra classes by students' entering math skills. As a result, it affected academic outcomes among students not targeted by the policy; test scores among high-ability students improved whereas their grades declined. (Contains 6 tables, 3 figures and 26 footnotes.)