Is anyone familiar with data that that has or could give provide a measure of school quality at the county level?
The Department of Education maintains rating data on schools in a large number of categories. You can build your own tables here:
The US Dept. of Ed also has related search tools here:
The link below will allow you to very a variety of aggregated data at state, county, school district and school level. Note, only a minimal amount of data is aggregated at county level.
For those that want to fully customize their results (datasets), use this tool: