Urban public school districts in the U.S. vary in quality. Educational policies, distribution of resources, and the challenges that students bring to the classroom differ substantially across localities. Little surprise, then, that academic outcomes--such as high school graduation rates and average school performance on standardized tests--fluctuate widely, too. This paper quantifies such variation in elementary and middle school quality within 68 of the largest U.S. public school districts, which collectively serve about 7.8 million students. It also examines whether cities that see students with certain demographics (specifically, low-income and nonwhite) concentrated in certain schools experience higher variation in school quality. The findings, especially those in the comprehensive Appendix table, can serve as a guide for parents, as well as an admonition to school officials in cities where quality is consistently low or where the quality gap between the best and worst schools is wide.