Did your kids moan that winter break was way too short as you got them ready for the first day back in school?

They might get their wish of more holiday time off under proposals catching on across the country to lengthen the school year.

But there’s a catch: a much shorter summer vacation.

Education Secretary Arne Duncan, a chief proponent of the longer school year, says American students have fallen behind the world academically.

“Whether educators have more time to enrich instruction or students have more time to learn how to play an instrument and write computer code, adding meaningful in-school hours is a critical investment that better prepares children to be successful in the 21st century,” he said in December when five states announced they would add at least 300 hours to the academic calendar in some schools beginning this year.

The three-year pilot project will affect about 20,000 students in 40 schools in Colorado, Connecticut, Massachusetts, New York and Tennessee.

Proponents argue that too much knowledge is lost while American kids wile away the summer months apart from their lessons. The National Summer Learning Association cites decades of research that shows students’ test scores are higher in the same subjects at the beginning of the summer than at the end.

“The research is very clear about that,” said Charles Ballinger, executive director emeritus of the National Association for Year-Round School in San Diego. “The only ones who don’t lose are the upper 10 to 15 percent of the student body. Those tend to be gifted, college-bound; they’re natural learners who will learn wherever they are.”

Supporters also say a longer school year would give poor children more access to school-provided healthy meals.

Yet the movement has plenty of detractors — so many that Ballinger sometimes feels like the Grinch trying to steal Christmas.

“I had a parent at one meeting say, ‘I want my child to lie on his back in the grass watching the clouds in the sky during the day and the moon and stars at night,’” Ballinger recalled. “I thought, ‘Oh, my. Most kids do that for two, three, maybe four days, then say, “What’s next?”’”

But opponents aren’t simply dreamy romantics.

Besides the outdoor opportunities for pent-up youngsters, they say families already are beholden to the school calendar for three seasons out of four. Summer breaks, they say, are needed to provide an academic respite for students’ overwrought minds, and to provide time with family and the flexibility to travel and study favorite subjects in more depth. They note that advocates of year-round school cannot point to any evidence that it brings appreciable academic benefits.

“I do believe that if children have not mastered a subject that, within a week, personally, I see a slide in my own child,” said Tina Bruno, executive director of the Coalition for a Traditional School Calendar. “That’s where the idea of parental involvement and parental responsibility in education comes in, because our children cannot and should not be in school seven days a week, 365 days a year.”

Bruno is part of a “Save Our Summers” alliance of parents, grandparents, educational professionals and some summertime recreation providers fighting year-round school. Local chapters carry names such as Georgians Need Summers, Texans for a Traditional School Year and Save Alabama Summers.

Camps, hotel operators and other summer-specific industries raise red flags about the potential economic effect.

The debate has divided parents and educators.

School days shorter than work days and summer breaks that extend to as many as 12 weeks in some areas run up against increasing political pressure from working households — 30 percent of which are headed by women. These families must fill the gaps with afterschool programs, day care, baby sitters and camps.

“Particularly where there are single parents or where both parents are working, they prefer to provide care for three weeks at a time rather than three months at a time,” Ballinger said.

The National Center on Time & Learning has estimated that about 1,000 districts have adopted longer school days or years.

Some places that have tried the year-round calendar, including Salt Lake City, Las Vegas and parts of California, have returned to the traditional approach. Strapped budgets and parental dissatisfaction were among reasons.

School years are extended based on three basic models:

• 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 last 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 1/2-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.