When I was a kid, I remember that "summer school" was for kids who were in danger of being held back. But, of course, there's more to it than that. Some districts offer summer enrichment programs (in, for instance, Mandarin). When I lived in South Bend, a highlight of each year was participating in the "Firefly" musical production run by the school corporation, which pulled the best singers and dancers from the entire district. This was, technically, a summer school course.
Unlike the federal government, many states require themselves to balance their budgets. That means that as the economy turns south, tax revenues dry up, and something has to go. Alas, this tends not to be the person assigned to make sure all highway signs reflect the new gubernatorial administration as soon as it changes over, or the person assigned to print out all emails for the Luddite assistant undersecretary for agriculture public relations. Legislatures instead tend to cut the enrichment programs. The New York Times has an article about this here. If any readers have seen enrichment programs hit the chopping block in their district, please let me know.