One in two US secondary teachers is planning to leave the profession within five years.
Overall, two in five teachers do not see themselves in a classroom beyond 2010, says a poll of 1,000 teachers across America released by the National Center for Education Information.
Retirement is a key factor in the impending exodus, with 42 per cent of teachers now almost 50 or older, compared to 24 per cent in 1996.
Emily Feistritzer, president of the Washington DC-based pollster, said an influx of older recruits is driving up the average age of teachers. One in three new teachers are "mature entrants" from other fields.
Susan Kardos, research fellow at Brandeis University's Mandel Center and senior research affiliate at the Project on the Next Generation of Teachers at Harvard University, said earlier studies had found a quarter to a half of new teachers were mid-career switchers. The average age of such recruits was 38, according to a 2003 Harvard poll. "The idea of a fresh-faced 22-year-old doesn't fit any more," Kardos said.
The trend has profound implications for schools, she added. "Someone coming in from being a lawyer will have different expectations about the workplace to someone straight from university."
The age shift follows the growth of "quickie" teacher qualification schemes - bypassing the costly, protracted traditional route into the profession via teacher-training colleges.
But such courses have had a mixed impact on staff turnover. Some 39 per cent of mid-career entrants are already in their 40s and 50s. But, Kardos noted, mature recruits "tend to make a longer commitment to teaching" than younger entrants, some of whom are "just trying it out", or think of it as service before embarking on another career.
Forty to 50 per cent of newly-qualified US teachers quit within five years, according to University of Pennsylvania research.
The report had more bad news about the dearth of male teachers: they made up just 16 per cent of staff with five years' experience or less.