School authorities across the nation are warning thousands of teachers that they could lose their jobs in June, raising the possibility that America’s public schools may see the most extensive layoffs of their teaching staffs in decades.

Though many of the warnings may not be acted upon — school systems, their budget outlook unclear, routinely overstate likely layoffs at this time of year — when layoffs do occur, they cause a chaotic annual reshuffling of staff members. Thousands of teachers are forced to change schools, grades or subjects, creating chronic instability that educators call “teacher churn.”

“Most districts have not done layoffs for years, so they have no idea how bad this is going to be when it hits,” said Timothy Daly, president of the New Teacher Project, a nonprofit group that has studied the effects of teacher layoffs.

Much of the public debate over teacher layoffs has concerned how they are decided, with sharp divisions between politicians and union leaders over the seniority-based layoff methods stipulated in union contracts.

Many argue that the rules rob schools of the talented young teachers who are the first to be let go. Union officials say that without such protections, more senior teachers would be let go first to save money.

But school superintendents say the consequences of sweeping layoffs are often overlooked in the policy debate. Layoffs, they say, hurt school cohesion, undermine student achievement and rupture ties with parents.

“I’m getting nauseous just remembering,” said Barbara Byrd-Bennett, who as the Cleveland superintendent in the last decade had to make draconian teacher cuts. “The result was devastating for our classrooms.”

School districts from Rhode Island to California have begun notifying teachers of layoffs. State laws or union contracts require notifications in the spring to teachers whose contracts might not be renewed.

In New York, Mayor Michael R. Bloomberg painted a worst-case outcome of 4,675 teacher layoffs last month. But the city may avoid many of those.

School finance experts say widespread teacher layoffs are more likely this year. The billions that Congress approved in 2009 and 2010 to forestall school layoffs is mostly spent. And, because their personal retirement accounts have dwindled during the recession, older teachers are delaying retirement, which means more layoffs would be needed to close budget gaps, said Marguerite Roza, a researcher at the University of Washington.

“It’s logical to anticipate much larger layoffs this year,” Dr. Roza said.

Randi Weingarten, president of the American Federation of Teachers, said criticism by politicians of seniority-based layoffs obscured that “this magnitude of cuts will hurt children, whether you lay off senior people or junior people.”

Arun Ramanathan, a former senior administrator in San Diego, said seniority-based layoff policies created more turmoil than was publicly recognized. “The public imagines that districts just lay off teachers starting at the bottom of a long seniority list,” Mr. Ramanathan said.

But the process is more complex in many districts, he said, partly because layoff quotas are first allocated to each of a district’s schools, and teachers laid off from one school have the right to take the job of junior educators at other schools, if they are certified to teach that subject.

“As a result, the harm of a single layoff can be multiplied, as a cascading process of ‘bumping’ begins,” said a February study by Education Trust-West, a nonprofit group, that was critical of California’s seniority-based layoff policies. Mr. Ramanathan was one of its authors.

“The system is quite insane,” he said.

Quyen Tran knows this turmoil well. A 30-year-old teacher in the New Haven Unified School District south of San Francisco, Ms. Tran has been pink-slipped in the spring four of the five years she has taught, but called back to teach every year. The layoffs and recalls have repeatedly forced her to refocus. She has taught five subjects and grade levels in three schools.

At the end of each of her first two years, Ms. Tran was bumped from her classroom by more senior teachers. Since getting tenure in her third year, she has twice bumped junior teachers from their classrooms. She received her fourth pink slip this month, and has attended California Teachers Association rallies to protest the layoffs.

Rick LaPlante, a spokesman for her district, said Ms. Tran’s career upheaval was “not common but not rare.”

“It’s chaos,” Mr. LaPlante said of California’s system of annual layoffs. “Maybe unfortunate is the best word. Or terrible.”

Those adjectives also describe the disruption layoffs caused in Minneapolis in the mid-2000s, according to a report by the New Teacher Project, which worked with the district.

As enrollments dropped from 2005 to 2009, Minneapolis schools shed 15 percent of their teachers, the report says. But each year half the teachers laid off in the spring were rehired in the fall. Half of those rehired returned to teach in a different school.

William Green, who was the Minneapolis superintendent from 2006 through last year, said parents felt victimized.

“Parents are scared to death when they send kids to school, and teachers become the face of the system for parents,” Dr. Green said. “We hurt relations with a lot of parents by laying off their child’s teacher.”

In Cleveland, Dr. Byrd-Bennett presided over even more severe teacher layoffs. During her seven-year tenure, she created dozens of theme schools, and student achievement began rising.

But in the years after the layoffs, achievement leveled off, and it eventually dropped. In 2009, school authorities invited the Council of the Great City Schools, a nonprofit group that represents urban districts, to visit Cleveland to diagnose problems.

The council’s report concluded that teacher layoffs, carried out by seniority, had stripped Cleveland’s specialty schools of key teachers. A Spanish-English immersion school had lost its dual language teachers; a school for gifted children had lost teachers who had special training to work with those students.

“The churn caused by layoffs can be extremely disruptive and hurt student achievement,” said Michael Casserly, the council’s executive director. “And conditions are ripe for disruptions to be dramatic this year.”