Wake County would have few peers among public school districts if it approves a plan to convert all schools to some form of year-round calendar.

More than 430 districts nationwide use types of year-round calendars, according to the National Association of Year-Round Education in San Diego. But only 81 of those districts use year-round calendars exclusively, and none is even half the size of Wake.

"Most districts that have year-round schedules also have traditional calendars," said Sam Pepper, the association's executive director. "It's not very efficient, but that's the mix you find."

Districts typically offer that mix because the year-round option often isn't popular with parents. So some offer year-round as a choice, while others resort to mandatory multitrack schedules to handle enrollment growth, as the Wake school system is proposing.

"You put the word mandatory in front of anything, and people don't like it as much -- and that's especially true of schools," Pepper said.

More students can attend a school that operates year-round on multiple tracks because the student body is divided into four groups. The groups' schedules are staggered so that at any given time, one group is on break and the other three fill the school.

Students attend classes 180 days a year, the same number as those on traditional calendars.

The National Association of Year-Round Education supports year-round schools for academic reasons, but its leaders concede that many districts consider the schedule only because of its ability to increase school capacity.

The public schools that serve Las Vegas and Los Angeles use the calendar for that reason.

In Clark County, where Las Vegas is located, elementary schools are automatically converted to year-round when they exceed capacity by a specified amount. Now, there are 70, but the district has 123 elementary schools on traditional calendars. Some schools have even been converted back to nine-month calendars when enrollment declined.

Los Angeles uses several types of year-round calendars at once, including one that reduces the school year to 163 days by increasing the length of each day.

"That can increase your capacity as much as 50 percent, but it makes a mess of the school year," Pepper said.

In districts that offer only year-round calendars, school officials say the approach works best when everyone is on the same schedule.

"Multiple tracks used to handle growth don't tend to work as well in the long run because parents don't have as much reason to buy into the calendar," said Terry Locke, a spokesman for the Chandler, Ariz., schools, a district of about 31,000 that uses a single-track schedule.

Marshall Bruni, president of the Austin Council of PTAs in Austin, Texas, agreed that regardless of a district's schedule, it works best when it's uniform. Austin experimented with year-round schools in the 1990s but abandoned the idea. Bruni, whose child attended a year-round school, said different schedules within one family were among the problems. "I don't have any problem with the idea, but you really need support from everyone to make them work," he said.