This from a Chicago public education newspaper called, "Substance" - only one of hundreds of "been there, done that" reports:

Valley View: An Illinois example of year-round failure
But Chicago policy makers could also have found damning evidence merely by researching the state’s own history with a year-round calendar, and especially the experiment in the Valley View school district during the 1970s, which ended after 11 years. Valley View was instrumental in the revival of the year-round school movement. The suburban Chicago school district made national headlines when in 1970 it became the first to use a year-round calendar district wide. By 1980, however, the district decided to drop it. Among the many reasons cited: higher utility costs. And that’s no small consideration in these days of escalating and erratic oil prices. School administrators also found no academic advantage to the calendar, said Emmie Dunn, administrative assistant to the superintendent. “As one of our administrators said when the schedule ended, ‘After 10 years of year-round school, you’re just plain tired,’ ” Dunn told one reporter.
Among those Illinois districts that have found the year-round school concept wanting are: Bloomington, Carlinville, Community Consolidated School District, Peoria, Quincy, Rockford, Valley View, Wesclin and Woodland.
Even Illinois parochial schools have shelved the idea. In 1998, a task force of the Roman Catholic Diocese of Peoria concluded the calendar would “unnecessarily complicate the lives of those who would participate in the educational change without sufficient benefit.”
A three-year evaluation study released in 1998 by Delwyn Harnish, a former professor of educational psychology at the University of Illinois at Urbana-Champaign, found no significant improvement in test scores at Kenwood Elementary, Champaign. He also found a significant jump in utility bills at Kenwood — from $27,580 the year prior to $41,537 after the calendar change. And this was during a time of relatively stable energy prices. It was a particularly embarrassing study for the year-round school movement in the state because the principal at that school, Les Huddle, was then vice president of the Illinois Association For Year-Round Education. The Kenwood study reflected earlier findings in Peoria, which dropped a pilot year-round school program in 1993 after three years of disappointing test scores.
Shortly thereafter, a study by a Bradley University economist warned one school district that mandating year-round school would result in a flight of families from those schools.
The year-round school movement has a long history of failure. Research by The Nation Education Association in a 1958 report found that every school system that had attempted a 12-month calendar up to that point eventually abandoned it. Valley View school officials who were among the founders of an organization that promotes school calendar change admit in a book that through 1968 every community which had either tried a year-round calendar or thoroughly investigated the idea had rejected it. The reasons communities dumped it decades ago are the same reasons they dump year-round school today: disruptive to family life, little or no academic benefit and little or no cost savings for schools—and it can even cost much more! Just recently, in neighboring Indiana, the Lafayette school district switched back to a traditional calendar after nine years of test scores revealed no significant academic value associated with a year-round calendar.