Almost a quarter of core subject classes in American secondary schools are taken by non-specialist teachers, according to an analysis of the department of education's latest staffing survey.
The revelation will come as a blow to White House efforts to have all English, maths, science and social studies lessons taken by highly-qualified teachers within three years.
President Bush's education legislation calls for new secondary teachers to have specialised as an under-graduate, or possess an equivalent qualification, in all subjects they teach.
By 2005, all staff must demonstrate "solid content knowledge of the subjects they teach" or their schools will lose funds.
A new study suggests the Bush administration will have its work cut out achieving the target.
Analysis of 1999-2000 data by schools think-tank, the Education Trust, found that 70 per cent of nine to 13-year-olds' maths classes were taught by staff who had not even studied the subject as a minor part of their degree.
And the situation is deteriorating. The latest survey shows that the proportion of "out of field" teachers increased from 21.8 per cent in 1993-4 to more than 24 per cent in 1999-2000.
Richard Ingersoll, University of Pennsylvania education professor, who compiled the study, said misassigning teachers had become a "standard" stop-gap measure in staff-strapped schools.
"You save time and money, but it is not cost-free to students. It is one of the reasons the US scores badly in international academic comparisons," he added.
But Gerald Tirozzi, executive director of the National Association of Secondary School Principals, said "it defied logic" that principals would assign teachers to other subjects "out of convenience" when schools are held accountable for pupils' scores in standardised tests.
Headteachers "play the hand they are dealt," he added. Staff retention is the biggest headache for US schools, despite the emphasis on boosting recruitment, according to another recent survey.
Nearly half of new recruits quit the profession within five years demoralised by low salaries, unruly pupils, dilapidated buidings and other factors, according to the National Commission on Teaching and America's Future.
Professor Ingersoll, who also conducted that research, said the new subject-qualification requirement "held an axe over schools" without addressing the staffing crisis.