School funding policies vary widely across the United States, but most states use formula funding to target districts serving low-income students. In this report, we present a new measure of the progressivity of school district funding, focusing on the degree to which the average low-income student attends districts that are better funded than districts the average non-poor student attends. We find that many states that have progressive funding formulas on paper do not achieve this goal in practice, and that, in some states, the potential progressivity of school funding is constrained by how low-income students are spread across school districts.