It looked like any other first day of school. Parents snapped photos of bleary-eyed children in freshly pressed shirts. Kids scanned class lists for their names and gave high fives to friends who were assigned the same teacher. The principal warmly welcomed everyone back for another year.

William Merritt, 5, was all smiles. On his first day of kindergarten at Alexandria's Samuel W. Tucker Elementary School, he recited rules his mother, Pauline, had drilled into him: "Play outside . . . and not punch people when they punch you," he said.

But for all its familiarity, one thing was very different this year: On the first day of school, classroom calendars were turned not to September but to July.

Last week, Tucker started its new year as Alexandria's first year-round school, cutting short students' and teachers' summer vacations by more than a month for a calendar that proponents say can help students retain material and, in turn, boost test scores.

Tucker students will attend the mandatory 183 days of school in four nine-week sessions separated by two- or three-week breaks. During the breaks, the school will offer optional remediation and enrichment classes -- adding as much as six weeks of instruction to the year, at a cost to the district of more than $365,000. Summer vacation will last one month.

"It really just continues the process of learning throughout the school year," said Tucker Principal Patrick McClintock-Comeaux. "We're doing it to meet a new level of excellence."

In recent decades, thousands of schools nationwide, including 11 others in Northern Virginia, have made similar changes to their calendars. Since 1989, the number of year-round schools has risen from about 600 in 19 states to more than 3,000 in 47 states, according to the National Association for Year-Round Education.

In 1998, Timber Lane Elementary in Fairfax County became the Washington region's first year-round school since Prince William County stopped using similar programs in the 1970s. Fairfax has since put 10 more schools on 12-month schedules. Last August, Barcroft Elementary became the first Arlington public school to switch to a year-round calendar. Other schools in the region are studying the idea.

But as the year-round trend has caught on, opposition has grown along with it. Critics say such schedules are expensive, wreak havoc with family lives and rarely produce higher test scores. In many school districts, including Prince William County and some Maryland districts in the 1990s, year-round proposals have met fierce resistance. Some school districts have tried the system and then scrapped it when parent support faded.

Alexandria school officials say there is no guarantee the year-round system will raise student achievement or work for families. But they say they are confident the calendar can be successful at Tucker -- even if it has failed elsewhere -- because the school has what some experts call the key ingredient to year-round success: overwhelming parent and staff support.

That support was evident July 26, Tucker's first day. Parents dropping off their children raved about year-round classes. Some said they felt relieved that they would no longer be forced to schedule family vacations during summer months. Others said they figured less pool and television time could only help their children learn.

"If he gets to go to school year-round, there's no lapse of memory," said Alba Thillet, a security officer, whose 10-year-old son, Brian, was starting fifth grade.

Students, brimming with first-day excitement, scarcely seemed aware their summer had lasted only six weeks. Third-grader Matthew Merritt, 7, said he had gone to the mall, learned how to dive and gotten a new model rocket.

"I've done everything I wanted to do this summer," said Matthew, who is William's brother.

The school's decision to go to a year-round, or "modified," school calendar, was based on two years of careful study and planning by staff and parents, McClintock-Comeaux said.

Alexandria School Superintendent Rebecca L. Perry first asked elementary schools to consider a year-round calendar in 2001. Tucker jumped at the chance, said Cathy David, who was principal of the school until 2003 and is now the district's director for elementary programs.

Unlike many schools that opt for a year-round calendar, Tucker officials were not looking to raise low student test scores. At Tucker, many of the 600 students come from low-income homes where English is not spoken, but students have excelled on achievement tests in the four years that the school has been open.

Tucker administrators and teachers thought the calendar might take their students even higher. "As long as there's any child not achieving, then our work is not done," David said.

Proponents say year-round schooling, which can take various forms, just makes sense. Redistributing the long summer vacation into regular, shorter breaks prevents students and teachers from burning out and keeps attendance high and discipline problems low, supporters say. Students also have less time to forget what they have learned.

Research on the scholastic benefits of year-round school is "not terribly powerful," said Harris Cooper, director of the Program in Education at Duke University, who has compared test results of schools on modified school calendars to those on traditional calendars at districts across the country. Test scores at year-round schools increase modestly, though students who are non-native English speakers or who are struggling in school often show more acute gains, he said.

Several studies have concluded that teachers are happier on year-round calendars, Cooper said.

"I think of [a modified school calendar] as a series of sprints," said Mary Ann Ryan, director of elementary instruction for Fairfax County Public Schools. "You sprint from one nine weeks to the next and then you catch up and you rejuvenate, whereas the traditional school calendar is more like a marathon. For most people the sprint is an easier chunk to manage."

According to NAYRE, about 17 percent of schools that go to a year-round calendar abandon it. Detractors say that's because the calendar's disadvantages outweigh benefits. Tina Bruno, executive director of the San Antonio-based Time to Learn, which opposes year-round education, said shorter summers prevent children from going to camps or spending time with noncustodial parents. Finding child care during mid-year breaks is far more difficult than during the summer, and logistics can be nightmarish for parents who have children in schools on different schedules, she said.

Year-round education can also be costly for school districts, which must pay for staff and transportation during intersessions -- the programs and courses held during the breaks -- and higher summer electricity bills, Bruno said.

"A lot of schools leave, and they leave with the same comments: No academic improvement and higher costs," she said.

McClintock-Comeaux said a committee of parents and staff looked closely at year-round schools in Fairfax County. Test scores in those schools have increased slightly, "but the anecdotal evidence is definitely strong. . . . Teacher satisfaction is higher; parent satisfaction is higher; attendance has improved; and discipline reports are down," he said. "Is it because of the calendar? You can't say that. But it does seem positive."

The committee liked that regular intersessions would give struggling students a chance to catch up and allow Tucker to offer the kinds of classes that many public schools have sacrificed in recent years, such as drama and foreign language.

And some of the common pitfalls of the year-round calendar do not apply to Tucker, school officials said. Most of the school's students come from families that cannot afford to send their children to summer camp. Some families had a hard time planning trips to visit relatives abroad during the summer and winter high seasons, when traveling is more expensive.

In surveys, more than 90 percent of parents and staff voted for the change. Tucker drew up a year-round plan for the 2003-04 school year, but the school board did not approve funding for the proposal until February.

The calendar is a hit with teachers, said second-grade teacher Kakie Sawyer, partly because students "know exactly what to do right away" on the first day of school -- without review. "And I'm up," she said. "I've had just enough rest."

The process has not been entirely smooth. Terrie Moody, president of the Tucker PTA, said she and some other parents with older children were initially concerned that the two-week breaks would be difficult to manage and that intersessions might not have space for all interested students. She said some parents were also miffed that the school did not give them final notice of the change until May.

Nevertheless, she said, "I do think there is trust in Tucker."

Three families and two members of the 100-person staff, both teachers, opted not to stay at Tucker because the year-round calendar did not fit their schedules, McClintock-Comeaux said.

He said the school, using the experience of Fairfax schools as a guide, has tried to anticipate problems that could arise under the new calendar and keep parents in the loop.

"I'm sure there will be 27 things we haven't thought of yet," he said. "If anybody can handle those 27 bumps, it's my staff."

Samuel W. Tucker Elementary School Principal Patrick McClintock-Comeaux talks to parents in the lobby before letting them take their children to their classrooms on the first day of school. Rachel Nemeth, 6, above, waits with her mother, Anna, before school starts. George Holoman, 9, below, a fifth-grader, waits in the lobby.Returning first-grade teacher Shawn Sponhauer, left, gives last-minute details to Amy Jordan, right, a new first-grade teacher.Tucker Principal Patrick McClintock-Comeaux assures Alba Thillet that her son, Brian, 10, is going to the right class. It's Brian's first day at Tucker.