—stretching the traditional 180 days of school across the whole calendar year by lengthening spring and winter breaks and shortening the one in the summer.
—adding 20 to 30 actual days of instruction to the 180-day calendar.
—dividing students and staff into groups, typically four, and rotating three through at a time, with one on vacation, throughout the calendar year.
At the heart of the debate is nothing less than the ability of America's workforce to compete globally.
The U.S. remains in the top dozen or so countries in all tested subjects. But even where U.S. student scores have improved, many other nations have improved much faster, leaving American students far behind peers in Asia and Europe.
Still, data are far from clear that more hours behind a desk can help.
A Center for Public Education review found that students in India and China — countries Duncan has pointed to as giving children more classroom time than the U.S. — don't actually spend more time in school than American kids, when disparate data are converted to apples-to-apples comparisons.
The center, an initiative of the National School Boards Association, found 42 U.S. states require more than 800 instructional hours a year for their youngest students, and that's more than India does.
Opponents of extended school point out that states such as Minnesota and Massachusetts steadily shine on standardized achievement tests while preserving their summer break with a post-Labor Day school start.
"It makes sense that more time is going to equate to more learning, but then you have to equate that to more professional development for teachers — will that get more bang for the buck?" said Patte Barth, the center's director. "I look at it, and teachers and instruction are still the most important factor more so than time."
The center's study also found that some nations that outperform the U.S. academically, such as Finland, require less school.
Many schools are experimenting with the less controversial, less costly interim step of lengthening the school day instead of adding days to the school year.
Chicago's public schools extended the school day from 5 hours and 45 minutes to 7 hours last year after a heated offensive by unionized teachers and some parents. Mayor Rahm Emanuel, former chief of staff to Duncan's boss, President Barack Obama, initially pushed an even longer school day — a major sticking point in this year's seven-day teachers' strike. He and other proponents argued that having the shortest school day among the nation's 50 largest districts and one of the shortest school years had put Chicago's children at a competitive disadvantage.
Wendy Katten, executive director of Raise Your Hand for Illinois Public Education, said opponents held back a push for a 7.5-hour school day, and got an extra staff person assigned to each school to handle the additional hour and 15 minutes of school time.
In San Diego, year-round school has been a reality since the 1970s.
District spokesman Jack Brandais said the concept was initially intended to relieve crowding, not improve performance test scores. The student body and staff were divided into four groups, with three attending school at any given time.
Through decades of fine-tuning, Brandais said the district now runs both traditional and year-round tracks simultaneously.
A 2007 study by Ohio State University sociologist Paul von Hippel found virtually no difference in the academic gains of students who followed a traditional nine-month school calendar and those educated the same number of days spread across the entire year.
Amid budget cuts and teacher layoffs, San Diego has cut five instructional days from both year-round and traditional schedules since last year.
The National Summer Learning Association: http://www.summerlearning.org
National Association for Year-Round School: http://www.nayre.org
Coalition for a Traditional School Year: http://schoolyear.info
National Center on Time & Learning: http://www.timeandlearning.org
National School Boards Association's Center for Public Education: http://tinyurl.com/88zauew
Center for Public Education: http://www.centerforpubliceducation.org