Principal Ann Reever of South Frederick Elementary School was eager to start the school year -- so eager that classes began Aug. 17 for her 400 students, nearly a week before the rest of Frederick County's schools and three weeks ahead of Labor Day.

The extra-early start was designed to build more days of teacher training into the calendar and boost performance among the students, 60 percent of whom come from low-income families, Reever said.

"We are not a failing school, but we have to make Adequate Yearly Progress this year," she said, referring to the federal No Child Left Behind Act and its requirement that public schools face state sanctions if their students' test scores don't improve for two years straight.

South Frederick's situation highlights how school calendars, which once followed a nine-month course from September to June, are stretched, patched and rearranged to accommodate the increasing demands for student achievement.

"You can't close the achievement gap [between high- and low-performing students] with the same amount of time as everybody else," Reever said. "Time was our variable we could work with."

Most Maryland school systems will start classes at least several days before Labor Day, which is next Monday. Anne Arundel, Howard, Montgomery, Charles and St. Mary's counties started Monday. Frederick and Prince George's started Aug. 23, and Calvert County began Aug. 24.

The school calendar, as defined by state law, must contain at least 180 class days (and at least 1,080 hours of instruction) during a 10-month period. School systems frequently adopt calendars extending beyond 180 days to provide a cushion when foul weather cancels classes.

It's different in Virginia, where a state law requires that schools start the Tuesday after Labor Day. That requirement, dubbed the "King's Dominion Law" for its supposed protection of the state's tourism industry, allows waivers so that some school districts can start earlier.

Crafting a system's calendar is a juggling act involving teachers, administrators, parents and students. Days must be fit in for religious observances, holidays and this year, the Nov. 2 election.

"It makes it very hard to do the best [schedule] pleasing the most people," said Kate Harrison, a spokeswoman for the Montgomery County school system. The process of agreeing on a starting date also must confront a mind-set that "we have the summer off," said Howard County parent Deborah Wessner. "Starting in mid-August gets into vacation time."

But the need to meet the standards of No Child Left Behind and to clear class time for Maryland's new High School Assessments is having more of a ripple effect throughout the school year, educators said.

"It's gotten so that you have to arrange school to meet those tests," said Jack R. Smith, deputy superintendent for Calvert County schools.

That's what happened in Prince George's County, where the school board agreed this year to have classes start a week earlier than in the past because standardized tests are given in March. "We could hit the ground running," said Prince George's school board Chairman Beatrice P. Tignor.