Arlington, Va. ? While it’s the start of the school year for most U.S. students, children at Barcroft Elementary have been at their desks for nearly a month – and they’re fine with that.

The suburban Washington school is among 3,000 across the nation that have tossed aside the traditional calendar for one with a shorter summer break and more time off during the rest of the year. The goal: preventing kids from forgetting what they have learned.

Barcroft’s principal, Miriam Hughey-Guy, pushed for the new calendar in hopes of boosting student achievement. She had read studies showing the toll a long summer break takes on what students remember.

Tests given to kids in the spring and fall show children generally slide in math and reading during the traditional summer break lasting 10 to 12 weeks, says Harris Cooper, director of the education program at Duke University. Both poor students and their wealthier counterparts lose math skills, and kids from low-income families also decline in reading.

Ron Fairchild, executive director of the Center for Summer Learning at Johns Hopkins University, says reconfiguring the school calendar simply makes sense.

“You would expect an athlete or a musician’s performance to suffer if they didn’t practice,” he said.

The number of schools on modified calendars with shorter summer breaks more than doubled in the last 15 years. Today, 46 states have schools operating on these calendars – up from 23 states in 1992.