You know how large organizations tend to be poorly run and non-responsive to their constituents? Well, large cities have large school districts, whereas suburbs have small school districts...
My impression is that the size of suburban districts varies greatly by state. Of those I am acquainted with, in Colorado, Maryland, and Virginia every county or county-equivalent city has one school district. In Pennsylvania there can be several school districts in a county, some small enough to have one high school. I think that New Jersey is very fragmented also.
On the west coast it seems like school districts often only cover a single city. Here are the lists for Washington, Oregon, and California. There are tons.