[peterbecich]

I disagree it is caused by sub-urbanization or zoning. Housing tracts are built with schools.

I would argue the primary cause is an attempt at "school optimization"; the choice to attend the "better" school over the closer school. Also, standardization of schools has decreased with the great variety of charter, private and magnet schools now, increasing the reasons to skip the local school. Personally, I think standardization is best. You could make the same argument about how people choose colleges.