Thanks to the vagaries of winter weather and the peculiarities of state law, students and teachers in Loudoun County are returning to class Monday while most of their counterparts across Northern Virginia spend another week sleeping in and savoring summer.

Loudoun Superintendent Edgar B. Hatrick III has long argued in favor of sending students to school in late August to maximize instructional time before spring standardized tests. Maryland and D.C. schools typically start in August.

But Virginia places strict limits on how school systems choose start dates, and Loudoun hasn’t been allowed to start this early since 2005.

Under an unusual and oft-challenged law, school systems can’t open before Labor Day unless they qualify for a waiver, usually based on the number of snow-related closures during the previous decade.

In recent years, a string of mild winters kept Loudoun from qualifying for an early start. But then came Snowpocalypse and Snowmaggeddon, blizzards that buried Washington under record-setting drifts in the winter of 2009-10 and kept children out of school for days on end.

Those snow days pushed Loudoun over the threshold, giving the 63,000-student system the opportunity to determine its schedule. The School Board voted unanimously in October to start classes one week before Labor Day.

“We believe that school boards, who represent their communities, should be empowered to make this fundamental decision about scheduling with any eye toward what suits the community,” Hatrick wrote in an e-mail. In Loudoun, he said, “students are ready to return to school by the end of August.”

Standing in the way of local autonomy is the Labor Day law. Often called the “Kings Dominion law,” it was passed in 1986 to protect the tourism industry — including the Kings Dominion theme park — from losing late-summer customers.

Over the years, legislators have tinkered with the law. This year, the General Assembly loosened it to enable waivers to be granted for certain school systems surrounded by those that routinely start early — such as Roanoke, enveloped by Roanoke County.

But tourism industry lobbyists and sympathetic lawmakers have beaten back efforts to repeal the law. The Virginia Hospitality and Travel Association calls preservation of the law one of its “2011 legislative accomplishments.”

Senate Majority Leader Richard L. Saslaw (D-Fairfax) said sending students back to school early would cut into Labor Day weekend’s substantial sales tax revenue, and he hasn’t heard a compelling argument for giving up those dollars.

“I’d like to have a better reason than someone looking me straight in the face and saying, ‘We should be able to if we want to,’ ” he said.

Loudoun officials argued that starting in August would give students more time to learn what they need to know for Advanced Placement and state Standards of Learning exams, which are generally administered in May or June.

“The more we can do to prepare our kids in advance of those tests, the better off they’ll be,” said School Board Chairman John Stevens (Potomac).

Starting early means schools close earlier in June. Julie West, a mother of two Loudoun students, welcomed that change, saying the weeks after standardized tests are usually light on academics anyway.

“It’s really all about partying at the end of the year,” West said. “The brains have shut down.”

Standardized tests have helped make an early start attractive to school leaders.

Besides Loudoun, 76 other school systems — more than half the 132 statewide — were granted waivers this year, many of them in mountainous areas. Fauquier, Clarke and Rappahannock counties, on the rural fringes of the Washington region, qualified for snow-day waivers and started school last week.

Elsewhere in Northern Virginia, one school in Arlington County and two in Alexandria are allowed to start early this year under a provision in the law granting waivers for “innovative and experimental” programs.

Arlington and Fairfax County officials said they want full power to choose when schools open. “All we’re asking for is flexibility,” said Fairfax schools spokesman Paul Regnier. “We’re not saying when we would start.”

Many parents are also willing to consider a shift. In January, a Fairfax schools survey of more than 10,600 parents founds that 64 percent favored starting classes one week earlier. Seventy-one percent of more than 12,500 Fairfax teachers agreed.

In Richmond this year, lawmakers introduced at least five bills to return more control over start dates to school boards. None were voted out of subcommittee, but Del. Kaye Kory (D-Fairfax), who sponsored one of the failed measures, said the “unprecedented” interest in overturning the Labor Day law represents the “beginning of what will ultimately be a groundswell” of support.

Kory and Dels. James M. LeMunyon (R-Fairfax) and Thomas A. “Tag” Greason (R-Loudoun) said they would renew efforts to change the school-calendar law next year.

“Why is Richmond telling each of the localities when they can or can’t start school? To me, that should be a local decision,” Greason said. “The localities should know better what the requirements are of their citizens, and one size doesn’t normally fit all.”