Mesa, the largest school district in Arizona, began phasing out certified librarians from most of its schools last year. In Spokane, Wash., the school district cut back the hours of its librarians in 2007, prompting an outcry among local parents. More than 90 percent of American public schools have libraries, according to federal statistics, but less than two-thirds employ full-time certified librarians.

Image Stephanie Rosalias fifth-grade classes in the library are about more than books. School librarians today give their students a grounding in Internet research skills. Credit... James Estrin/The New York Times

Lisa Layera Brunkan, a mother of three in Spokane, said she recognized the importance of the school librarian when her daughter, who was 7 at the time, started demonstrating a PowerPoint project. “She said, ‘The librarian taught me,’ ” Ms. Brunkan recalled. “I was just stunned.”

School librarians still fight the impression that they play a tangential role. Ms. Rosalia frequently has her lessons canceled at the last minute as classroom teachers scramble to fit in more standardized test preparation. Half a fifth-grade class left in the middle of a recent session on Web site evaluation because the children were performing in a talent show.

“You prepare things to proceed in a logical sequence and then here comes a monkey wrench,” Ms. Rosalia said. “We are teaching them how to think. But sometimes the Board of Ed seems to want them to learn how to fill in little bubbles.”

In New York City, Ms. Rosalia is a relative rarity. Only about one-third of the city’s public schools have certified librarians, and elementary schools are not required to have them at all.

Ms. Rosalia ran beauty salons with her husband and volunteered in her sons’ school libraries before pursuing her graduate degree. She was recruited to P.S. 225 by Joseph Montebello, the principal, a brother of a middle school librarian in Brooklyn.

In the school, just a block from a bustling stretch of Brighton Beach Avenue with its overflowing fruit stands and Russian bakeries, Ms. Rosalia faces special challenges. More than 40 percent of the students are recent immigrants. Language barriers force her to tailor her book collection to readers who may be in seventh grade but still read at a second-grade level.