School systems are increasingly working to foster more ambitious learning environments and disrupt institutionalized educational inequities. But such efforts are complicated by a number of uncertainties--about the aims of schooling and how they might be achieved or measured--that suffuse educational organizations. Although there is general agreement that uncertainty is both pervasive and consequential in educational settings, as a field we lack a more unified analytic framework for making sense of this uncertainty. In this article, I seek to advance such a project by drawing on educational and organizational scholarship to identify three forms of uncertainty that pervade school systems. While two of these forms of uncertainty--environmental and technical--are well established in organizational theory, I develop the concept of representational uncertainty as a third form of uncertainty that is particularly impactful in schools. I show how these three forms of uncertainty manifest across different contexts, aims, and levels of school systems; how they interrelate; and how they must be strategically mitigated or fostered in the service of educational improvement.