This paper is the first to estimate the cost function of Connecticut public K-12 education and to evaluate the state's school spending based on regression-estimated education costs. It finds large disparities across districts in education costs and cost-adjusted spending. A large percentage of the state's public school students are enrolled in districts where spending is inadequate relative to the predicted cost of achieving a common student performance target. Thus, many school districts, especially the high-cost ones, need a large amount of additional spending to improve student performance.