WASHINGTON — A year is a long time in a child's education, the time it can take to learn cursive writing or beginning algebra. It's also how much time kids might spend with substitute teachers from kindergarten through high school — time that may be all but lost for learning.

Despite tremendous pressure on schools to increase instruction time and meet performance goals, the vacuum created by teacher absenteeism has been largely ignored. (It's been estimated that about 1,000 substitute teachers are in Hawai'i public school classrooms on any given day.)

New research suggests it 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 that kids are supposed to have all year — is up dramatically.

Duke University economist Charles Clotfelter, who has studied the issue, says the image of spitballs flying past a daily 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 a 2002 education law penalizes schools if too few students meet test-score benchmarks.

Though long-term substitutes often have better credentials than those chosen for daily fill-ins, they have not gone through the normal hiring process that scrutinizes regular, full-time teachers.

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 show that more than a fifth of public schools use subs in this way.

One factor was a rise in the number of schools reporting they had full-time vacancies. That points to teacher shortages in some communities.

Also, schools are being more thorough in reporting on vacancies and on school staffing in general because of requirements of the No Child Left Behind law.

Standards for substitutes vary widely but are typically well below those for full-time regular teachers. Some states and local districts don't require background checks, and many don't require substitutes to have attended college, let alone have graduated.

And states with the fewest standards for substitutes also rely most on subs. Principals in Arkansas, Louisiana, Mississippi, Tennessee and Washington, D.C., are most likely to identify teacher absenteeism as a big problem, according to Education Department survey data from 2003-04, the most recent available.

Among those places, only Washington requires all substitutes to have some college education. And even there, principals sometimes ignore that requirement when faced with teacher absences, according to a district review.

At Baltimore Polytechnic Institute, a high school with a math and science focus, a substitute might be in a math class one day and an art or science class the next, said principal Barney Wilson.

"We're not expecting him to teach the material. We're expecting him just to follow the lesson plan that the teacher laid out," Wilson said.