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.

Obama and Duncan say kids in the U.S. need more school because kids in other nations have more school.

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 the U.S. (180 days).

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.

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

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.

Disadvantaged kids, on the whole, make no progress in the summer, said Karl Alexander, a sociology professor at Baltimore’s Johns Hopkins University, home of the National Center for Summer Learning. 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.