In 2003, Chicago launched "Double-Dose Algebra," requiring students with pretest scores below the national median to take two periods of math--algebra and supplemental coursework. In many schools, assignment to Double Dose changed the peer composition of the algebra classroom. Using school-specific instrumental variables within a regression-discontinuity design (RDD), we find that attending a lower skill classroom reduced math achievement for median-skill students. As a result, the Double-Dose policy had little or no effect for median-skill students in schools that exposed them to low-skill classrooms. However, the effects of Double Dose were substantially positive in schools that did not do so. We consider policy implications and interpretations of the results from RDDs.