WASHINGTON – Students beware: The summer vacation you just enjoyed could be sharply curtailed if President Barack Obama gets his way.

Obama says American kids spend too little time in school, putting them at a disadvantage with other students around the globe.

"Now, I know longer school days and school years are not wildly popular ideas," the president said earlier this year. "Not with Malia and Sasha, not in my family, and probably not in yours. But the challenges of a new century demand more time in the classroom."

The president, who has a sixth-grader and a third-grader, wants schools to add time to classes, to stay open late and to let kids in on weekends so they have a safe place to go.

"Our school calendar is based upon the agrarian economy and not too many of our kids are working the fields today," Education Secretary Arne Duncan said in a recent interview with The Associated Press.

Fifth-grader Nakany Camara is of two minds. She likes the four-week summer program at her school, Brookhaven Elementary School in Rockville, Md. Nakany enjoys seeing her friends there and thinks summer school helped boost her grades from two Cs to the honor roll.

But she doesn't want a longer school day. "I would walk straight out the door," she said.

Domonique Toombs felt the same way when she learned she would stay for an extra three hours each day in sixth grade at Boston's Clarence R. Edwards Middle School.

"I was like, `Wow, are you serious?'" she said. "That's three more hours I won't be able to chill with my friends after school."

Her school is part of a 3-year-old state initiative to add 300 hours of school time in nearly two dozen schools. Early results are positive. Even reluctant Domonique, who just started ninth grade, feels differently now. "I've learned a lot," she said.

Does Obama want every kid to do these things? School until dinnertime? Summer school? And what about the idea that kids today are overscheduled and need more time to play?

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Obama and Duncan say kids in the United States need more school because kids in other nations have more school.

"Young people in other countries are going to school 25, 30 percent longer than our students here," Duncan told the AP. "I want to just level the playing field."

While it is true that kids in many other countries have more school days, it's not true they all spend more time in school.

Kids in the U.S. spend more hours in school (1,146 instructional hours per year) than do kids in the Asian countries that persistently outscore the U.S. on math and science tests — Singapore (903), Taiwan (1,050), Japan (1,005) and Hong Kong (1,013). That is despite the fact that Taiwan, Japan and Hong Kong have longer school years (190 to 201 days) than does the U.S. (180 days).

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Regardless, there is a strong case for adding time to the school day.

Researcher Tom Loveless of the Brookings Institution looked at math scores in countries that added math instruction time. Scores rose significantly, especially in countries that added minutes to the day, rather than days to the year.

"Ten minutes sounds trivial to a school day, but don't forget, these math periods in the U.S. average 45 minutes," Loveless said. "Percentage-wise, that's a pretty healthy increase."

In the U.S., there are many examples of gains when time is added to the school day.

Charter schools are known for having longer school days or weeks or years. For example, kids in the KIPP network of 82 charter schools across the country go to school from 7:30 a.m. to 5 p.m., more than three hours longer than the typical day. They go to school every other Saturday and for three weeks in the summer. KIPP eighth-grade classes exceed their school district averages on state tests.

In Massachusetts' expanded learning time initiative, early results indicate that kids in some schools do better on state tests than do kids at regular public schools. The extra time, which schools can add as hours or days, is for three things: core academics — kids struggling in English, for example, get an extra English class; more time for teachers; and enrichment time for kids.

Regular public schools are adding time, too, though it is optional and not usually part of the regular school day. Their calendar is pretty much set in stone. Most states set the minimum number of school days at 180 days, though a few require 175 to 179 days.

Several schools are going year-round by shortening summer vacation and lengthening other breaks.

Many schools are going beyond the traditional summer school model, in which schools give remedial help to kids who flunked or fell behind.

Summer is a crucial time for kids, especially poorer kids, because poverty is linked to problems that interfere with learning, such as hunger and less involvement by their parents.

That makes poor children almost totally dependent on their learning experience at school, said Karl Alexander, a sociology professor at Baltimore's Johns Hopkins University, home of the National Center for Summer Learning.

Disadvantaged kids, on the whole, make no progress in the summer, Alexander said. Some studies suggest they actually fall back. Wealthier kids have parents who read to them, have strong language skills and go to great lengths to give them learning opportunities such as computers, summer camp, vacations, music lessons, or playing on sports teams.

"If your parents are high school dropouts with low literacy levels and reading for pleasure is not hard-wired, it's hard to be a good role model for your children, even if you really want to be," Alexander said.

Extra time is not cheap. The Massachusetts program costs an extra $1,300 per student, or 12 percent to 15 percent more than regular per-student spending, said Jennifer Davis, a founder of the program. It received more than $17.5 million from the state Legislature last year.

The Montgomery County, Md., summer program, which includes Brookhaven, received $1.6 million in federal stimulus dollars to operate this year and next, but it runs for only 20 days.

Aside from improving academic performance, Education Secretary Duncan has a vision of schools as the heart of the community. Duncan, who was Chicago's schools chief, grew up studying alongside poor kids on the city's South Side as part of the tutoring program his mother still runs.

"Those hours from 3 o'clock to 7 o'clock are times of high anxiety for parents," Duncan said. "They want their children safe. Families are working one and two and three jobs now to make ends meet and to keep food on the table."

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Associated Press writer Russell Contreras in Boston contributed to this report.