Mary Waldman began her career teaching kindergartners how to hold a pencil and write their ABCs. Fifteen years later, she is teaching Loudoun County students to read books and write stories. While academic expectations have grown exponentially over the years, the length of the school day has stayed the same: Three hours.

She makes the most of her half-day class. Recess is not required. She rarely sets up paints or musical instruments or puppets. Some kindergarten teachers stopped offering a snack, because it takes up too much time. Waldman draws the line there.

“They are 5 and 6,” she said. “They need food so they can power through.”

About 75 percent of kindergartners nationwide are enrolled in full-day programs, three times the rate of a few decades ago, as many school districts have come to view kindergarten as an academic starting point, rather than a practicing ground for the rhythms and routines of school. But that leaves about a million students for whom kindergarten still lasts just a few hours a day.

That partial day represents a time crunch as teachers try to keep up with more challenging lessons. Rigorous Common Core standards are rolling out in 46 states, applying uniform expectations for the first time in kindergarten days that range from two to seven hours long. Virginia did not adopt the national approach, but the state has raised its own standards.

“It’s like saying, ‘I want you to prepare for a marathon, but you can only train for half the time,’” said Kyle Snow, director of the Center for Applied Research at the National Association for the Education of Young Children.

Loudoun County is the only district in the Washington region and one of just four in Virginia that does not offer universal full-day kindergarten. Other districts are pushing resources and attention to preschool , reflecting a national effort — which President Obama and his Education Department have endorsed — to expand access to early education based on research showing that quality early learning promote success across a broad spectrum.

Some Loudoun parents view the issue as one of equity: Their children aren’t getting the same preparation as other students, but they are held to the same standards, and they have to pay thousands of dollars extra if they want to have their children attend private schools for the other half of the day. Loudoun County officials believe their students emerge from half-day kindergarten doing just as well as any others, and they cite the prohibitive expense of expanding kindergarten at a time of tight budgets.

“I hear that today’s kindergarten is yesterday’s first grade,” said Lindsay Weissbratten, an Ashburn mother who started a parent group to advocate for universal full-day kindergarten. “Loudoun children deserve to have the same education as the rest of the state.”

The District offers preschool to all its 3- and 4-year-olds, and Arlington, Montgomery and Prince George’s counties offer full-day pre-kindergarten classes to all children from low-income families.

State policies leave school districts wide discretion to define the kindergarten day, according to a 2013 analysis by the Children’s Defense Fund . Ten states, including Maryland, as well as the District, require full-day kindergarten. Six states don’t require school districts to provide kindergarten at all.

State Sen. George L. Barker (D-Fairfax) introduced a bill in Richmond this year that would require full-day kindergarten statewide. But the bill failed in the Senate Education and Health Committee on an 8 to 7 vote. He said in an interview that Loudoun lobbied hard against the proposed mandate.

“Unfortunately, Loudoun County, the richest county in America, has turned its back on its children and families,” he said in a statement, noting that last year no school in the county provided a full-day program to all of its kindergarten students.

Sharon Ackerman, Loudoun’s assistant superintendent for instruction, said many of the county’s students come to kindergarten well-prepared by their families and private preschools, so the focus has been on bringing a longer day to those with the “greatest needs.”

Loudoun offers full-day programs to about a third of its 5,000 kindergartners, most of them English learners who get an additional half-day of language instruction. About 120 students, including graduates of Head Start, attend regional full-day kindergarten programs, and this year the district began offering full-day kindergarten at four high-poverty schools.

The School Board’s proposed budget for next year includes five more schools with similar demographics.

Loudoun officials say extra classrooms and teachers for full-day kindergarten would be too pricey, particularly given the already steep costs associated with keeping pace with the county’s growth. A 2012 analysis by the school system found it would cost as much as $65 million to nearly double its kindergarten space and staff to accommodate full-day classes for everyone.

Fairfax County took more than a decade to ease the financial impact and phase in full-day kindergarten. But fast-growing Prince William County made the leap in about four years, an expedited effort that Superintendent Steven L. Walts called his “greatest accomplishment.”

“I feel like I did every subsequent grade level a favor,” Walts said.

[READ: Universal full-day kindergarten my greatest accomplishment]

Some School Board members in Loudoun said they don’t see an educational need to extend the program countywide in the high-performing, well-to-do community.

“If you look at the test scores for our kids in this district, we do as fine or better,” said board member Kevin Kuesters of Broad Run.

He cited research that shows students in full-day programs get a boost academically in early years compared to those in half-day programs, but they don’t maintain their edge over time.

Early-education advocates say the benefits of full-day programs go beyond test scores. The longer days have been tied to better attendance, enhanced social and emotional development and have reduced the need for future retention and remediation . Children who come from poverty tend to benefit the most, “but it’s not an either-or,” said MaryLee Allen, acting policy director at the Children’s Defense Fund.

Waldman calls her kindergartners at Creighton’s Corner Elementary “busy bees.” Colorful cutouts of bumble bees hover over the workstations that her class cycled through on a recent Friday.

Outfitted with hole punchers, glue sticks or No. 2 pencils, the children practiced some of the skills they have been learning, including telling time, counting by fives, counting by tens, comparing the first and 16th presidents of the United States, writing sentences about their favorite healthy foods and identifying equations or numbers that are greater than or equal to 100.

“Whew! We’ve been busy today!” Waldman said as the students gathered on the carpet to review plans for the coming week. They had already read a poem and a story together. Next she sent them scurrying for their color-coded book bags, so they could read independently before they collected their coats and got ready to leave.

The teachers cover most of the academic standards in the partial day. “We use the three hours bell-to-bell for, particularly, literacy,” Ackerman said.

What is missing is the chance to go deeper into lessons and enough time to learn through play and exploration, which experts say is developmentally appropriate for younger students.

“If you are forced to spend so much time on heavy academics, that is not quality early childhood learning,” said Fairfax School Board member Jane K. Strauss of Dranesville, who cited this as a key reason the district adopted a universal full day. “You don’t want kids being forced to sit still and accomplish tasks for which they are too young.”

Parent Kimberly Murray said she was “stunned” that Loudoun did not offer full-day kindergarten when she moved from Vienna, where her two older sons had gone through a full-day program. The stay-at-home mother was concerned that Loudoun’s program would not be as challenging.

“He was reading before he started kindergarten,” she said of her youngest son, Carter. “He didn’t need to practice. He was definitely ready for a full day.”

So after morning kindergarten finished at 10:30, 6-year-old Carter took his place in a long line of students who supplement their public kindergarten experience with another half day at a private school.

The kiss-and-ride outside his Ashburn school was filled with white vans and school buses bearing the names of private schools, tutoring centers and tae kwon do academies. Carter boarded the bus for Golden Pond, a small brick school tucked behind a strip mall 15 minutes away.

The afternoon kindergarten enrichment program, which costs about $6,000, starts with recess. Then comes lunch.

“We are a loud room,” said Carter’s afternoon kindergarten teacher, Maggie Gearing, over the din of Pizza Friday. “We know they’ve been sitting all morning; they need to just burn off some energy.”

After lunch, the children typically have Spanish or music and lots of time for group art projects and other hands-on lessons based loosely on the state’s Standards of Learning. Then there’s another recess.

It’s like what kindergarten used to be “before it became teaching to the test,” said admissions officer Margaret Grace.

That Friday afternoon, Carter’s class hosted a “fairy tale tea” with cookies and lemonade for parents to celebrate the end of a month-long unit on fairy tales. The students dressed up like knights and princesses with hand-made shields and crowns and sang “Fee Fi Fo Fum” while their parents recorded it on smartphones.

Waldman said her students often tell her about what they do in their afternoon classes, and she mentally stores them in the category of Things They Used To Do — like playing dress up and tying shoes.

“Now we are like . . . there’s no time,” said her assistant, Diana Rigley. “Just keep going!”

Rachel Weiner contributed to this report.