Fierce local debates throughout the United States surround the equity of admitting students to public schools using academic criteria. Although research has evaluated the central assumption of these debates-that Selective Enrollment Public (SEP) schools enhance the welfare of students who attend them-none has addressed the district-level outcomes associated with these schools. This is important because the selectivity and scope of SEP schools produce tiered school systems (SEP districts). This district-level process, in turn, calls for an analysis of district-level achievement outcomes. To address this gap, I compile an original list of SEP schools using an innovative web scraping procedure. I combine these data with newly available district-level measures of third to eighth grade achievement from the Stanford Education Data Archive. Analyses follow a difference-in-differences design, using grade level as the longitudinal dimension. This approach facilitates a falsification test, using future treated districts, to reject spurious causation. I find evidence of overall slower growth in mean math achievement in SEP districts and for white, black, and Latinx racial/ethnic groups separately. SEP districts also see an increase in the white-Latinx math achievement gap. This work highlights the importance of considering SEP schools as part of a differentiated school system.