Coyla Lockhart, a retired teacher, talks with Alexis Kriegh, 8, at Hillcrest School. Lockhart is now used by the Lawrence school district as a substitute teacher almost every day.

Coyla Lockhart, who retired in 2006 after teaching for 23 years, continues to be a familiar face in Lawrence.

She’s a popular substitute teacher, and was back in her old stomping grounds at Hillcrest School last Friday. The day before she was at Central Junior High School.

Lockhart is one of about 65 substitute teachers Lawrence schools call upon each day to fill in for those who are ill, have illnesses in their families or are attending training, said David Cunningham, division director for human resources of certified personnel.

That’s 7 percent of the teacher work force.

The district has 924 teachers, of which 70 are part time. A pool of about 110 substitutes are available.

Lawrence principals say they are fortunate to be able to rely on Lockhart and other area substitutes who have extensive classroom experience. Nationally, finding experienced substitutes has become a concern.

New research shows that a child can spend up to a year with substitute teachers from kindergarten through high school. This, according to a national report, can have an adverse effect in the classroom.

The problem isn’t just with teachers home for a day or two with the flu. Schools’ use of substitutes to plug full-time vacancies – the teachers whom students are supposed to have all year – is up dramatically nationwide.

Duke University economist Charles Clotfelter, among a handful of researchers who have studied the issue, says the image of spitballs flying past a substitute often reflects reality.

“Many times substitutes don’t have the plan in front of them,” Clotfelter said. “They don’t have all the behavioral expectations that the regular teachers have established, so it’s basically a holding pattern.”

Clotfelter’s examination of North Carolina schools is part of emerging research suggesting that teacher absences lead to lower student test scores, even when substitutes fill in. And test scores have gained heightened importance, because the 2002 education law penalizes schools if too few students meet testing benchmarks. The goal is to get all students reading and doing math at their grade levels by 2014.

Raegen Miller, a postdoctoral fellow at the University of Washington, is examining the impact of teacher absences on fourth-grade test scores in a large, urban school district that he chooses not to identify. His findings show that 10 teacher absences within a year cause a significant loss in math achievement. When the regular teacher is gone for two weeks, it can set students back at least that amount of time.

“Teachers often have to reteach material, restore order and rebuild relationships after absences,” said Miller, who is conducting the research with Harvard University education professors.

The potential harm multiplies when subs are used in long-term roles in a classroom. Though long-term substitutes often have better credentials than those chosen for daily fill-ins, they are no replacement for regular, full-time teachers who have gone through the normal hiring process.

Nationwide, the number of schools reporting that they used substitutes to fill regular teaching vacancies doubled between 1994 and 2004, according to Education Department data. The latest data showed more than a fifth of public schools use subs in this way.

This year Lawrence is relying on mostly former or retired teachers to fill seven teacher vacancies, for the most part because administrators were unable to hire people for those positions to start the year, Cunningham said. Five of the vacancies are in special education.

District leaders want to be able to fill those positions, but they also believe retired teachers make good long-term substitutes.

“That is the difference that allows us to feel comfortable that we’re not going to suffer academically,” Cunningham said.

The district has been pleased with its evaluations of substitute teachers, he said.

Felton Avery, principal at Kennedy School, 1605 Davis Road, said it can be difficult if more inexperienced substitute teachers have to take a class for the day, although many retired teachers or more-experienced substitutes usually fill in.

For the most part, teachers leave detailed lesson plans, and administrators and other teachers also try to lend a hand to substitutes, he said.

“We’ve got some pretty dependable people there, so that makes a big difference,” he said.