Moira McQuillen thought that kindergarten would be an easy transition for her daughter this fall--no more than three hours a day at school.

But when she went to Arlington's Jamestown Elementary School to register the 4-year-old, she discovered that half-day kindergarten will no longer be offered at Jamestown or any other school in the county. Her daughter, who turns 5 in July, will be in class for 6 1/2 hours, just like children in the older grades.

"Although she probably could do it, there's such a high price to pay," said McQuillen, a stay-at-home mother whose daughter has gone to a preschool two days a week during the past year. "I imagine her coming in at 3:30, falling out in a weeping ball and not being able to wake her for dinner."

Now McQuillen has become one of the leaders of an effort to persuade the Arlington School Board to change its plans to eliminate half-day kindergarten this fall. She and several other parents say that although the full-day program is fine for many youngsters, some kindergartners aren't ready for it and should have the option of a shorter day.

School officials say that they don't have the money to offer both kinds of schedules and that full-day kindergarten is the best choice for most children.

The same issue confronts other Washington area school districts that are moving toward full-day kindergarten. Fauquier County, Alexandria and the District already have full-day classes at all of their elementary schools, and Prince George's County is in the process of converting its programs from half-day to full-day. This fall, 32 Fairfax County schools will offer full-day kindergarten, up from six this year. Montgomery County offers the classes at nine schools.

Parents generally like the full-day programs in the districts that offer them, both for the educational benefits and for the easing of child-care problems. In Arlington, which began phasing out half-day kindergarten in 1993, school officials say that the overwhelming majority of parents want the longer schedule. Several studies have shown that full-day kindergarten leads to better performance in later grades, especially among low-income children at risk of academic failure.

"We looked at the research and listened to our educators and determined that the full-day program was the best option for most of our population," said Arlington School Board member Frank K. Wilson. "I don't really know whether we are willing to go back and change things midstream."

School Superintendent Robert G. Smith initially proposed to create a half-day kindergarten magnet program at one school this fall, but the plan fell victim to budget cuts.

McQuillen and other critics of the board's decision say they understand the benefits of full-day kindergarten and support its continuation. But they argue that their children, too, should have access to programs that best meet their needs.

Some of them are stay-at-home parents who say they can offer better educational opportunities for their children during those extra hours. Others are working parents who say their children are too young for such a heavy dose of academics.

"For children raised in a home where one parent has chosen to stay home as the primary caregiver, the act of going to kindergarten is a transition," said John Pollard, a stay-at-home Arlington father whose 3 1/2-year-old son will enter kindergarten in two years. "Being stuck in a full-day program is an abrupt change for them that can cause a lot of anxiety."

About 20 parents went to a recent board meeting to urge the board to reconsider, and McQuillen said she so far has gathered 125 signatures on a petition.

The parents also have the support of Arlington County Board member Mike Lane (R). "It seems to me that the School Board is discriminating against parents who chose to stay home and educate their children," Lane said.

Parents supporting the half-day option have suggested that schools group all the kindergartners together and release the half-day students earlier, making parents responsible for picking them up. But school officials say it isn't that simple. A half-day program must have a different curriculum if it is to be effective, they say.

School Board member Mary H. Hynes said she thinks teachers could work around those difficulties and is prepared to reconsider the board's decision.

"Personally, I have been supportive of continuing the option," Hynes said. "I think to the extent that there may be clusters of parents at particular schools who want this, there's a possibility that we could accommodate them."