The authors sought to determine if an urban school district's effort to fundamentally change teaching, assessment, and intervention practices increased student achievement and decreased achievement variation among classrooms in 11 high schools. They examined Grade 11 mathematics achievement data from the 2009-2010 and 2010-2011 Kentucky Core Content Test. The sample consisted of 2,451 students in 11 high schools that implemented an initiative called Project Proficiency. The study used hierarchical linear modeling to determine if changes in instructional practices yielded academic gains, while controlling for individual and school socioeconomic status (SES) and prior student achievement. The analysis revealed that Project Proficiency had a significant impact on state mathematics achievement and reduced classroom variation by 55% in the final model, as opposed to 16% in the control group. Results also indicated that a school's SES had a greater influence on student achievement than an individual student's SES.