Adding 10 minutes to the school day may seem like a minor change.

But Pittsburgh Public Schools officials are hoping it will give them a new opportunity to show that every minute counts in the race to raise student achievement.

Some researchers and advocates have suggested that school districts moving to extended days use the extra time for individual or small-group instruction or for longer classes in core subjects. They said districts might consider redesigning the school day and re-evaluating priorities so extra time is used as effectively as possible.

Because they hadn't yet briefed the school board, district officials last week declined to reveal their plans for the additional minutes, part of the new contract with the Pittsburgh Federation of Teachers.

PFT President John Tarka said he believes the extra time will be used for instruction, as opposed to teacher training or other purposes.

"The fact of the matter is, 10 minutes, if used wisely, can be of benefit to the learning process," he said.

District officials have noted that the change, in essence, adds a week to the school year.

But Elena Silva, senior policy analyst at the Washington, D.C.-based think tank Education Sector, wasn't impressed.

"No, I don't think you can do a lot with 10 minutes," she said.

A longer school day is becoming a more popular concept nationwide, particularly among urban districts trying to salvage failing schools. Some of the leading proponents of more class time are Massachusetts education advocates who worked with Pittsburgh school Superintendent Mark Roosevelt when he was a legislator and activist in that state.

In the past three years, 18 schools received state funds to increase their school days by at least 25 percent, a figure that translates to about one hour and 40 minutes of extra time, according to Massachusetts 2020, a group that has promoted the effort.

"Some of the schools have added more. The Boston schools, for example, have added 33 percent more time," said Jennifer Davis, president of Massachusetts 2020 and the National Center on Time and Learning. Both nonprofit groups are based in Boston.

Ms. Davis said schools participating in the Expanded Learning Time initiative are showing early progress and noted that another 75 schools plan to join the effort in the next two years.

But more time isn't a panacea. The Pittsburgh district struggles with low achievement even though it already has a longer school day and longer school year than the 6¹/₂-hour, 180-day national average.

The Pittsburgh district's 2003 teachers contract set the school day at seven hours, six minutes. That increased the day by 16 minutes at high schools and by 11 minutes at other schools.

The 2006 contract established an eight-hour day at the eight accelerated learning academies. Of the 54 additional minutes, 45 minutes were allocated for instruction and the balance for staff training.

That contract also provided for 192 instructional days in an academy year, 10 more days than at other district schools.

The new contract, ratified by the union and school board in January, leaves the school day and year at the academies as is. But the day at other schools will increase to seven hours, 16 minutes, beginning in 2008-09.

In a study last year, Dr. Silva said "improving the quality of instructional time is at least as important as increasing the quantity of time in school."

She recommended that districts reflect on how they use school time. To make the best use of time, Mr. Tarka said, classrooms must be orderly,

The class-time numbers don't tell the whole story. The district offers after-school programs that keep students in class after the final bell.

Some Massachusetts schools have blurred the line between the school day and after-school programs, bringing in community organizations to offer enrichment while teachers have training.

The Massachusetts initiative requires schools to add at least 25 percent more time to the school day because officials determined that's an important threshold for achievement gains, Ms. Davis said. She was careful to avoid criticizing the 10 minutes Pittsburgh will add, however, saying careful planning can help to make the time worthwhile.

Dr. Silva recommends that schools consider adding 30 percent more time, or about two hours to the average 6¹/₂-hour day. "It's not too much," she said. "It's not too little."