Government proposals to raise the bar for trainee teachers in England could worsen the shortage of science and maths teachers, a report suggests.
Education Secretary Michael Gove has said the government should only pay to train graduates as teachers if they have at least a 2:2.
A study by Buckingham University says that policy would lead to “big holes” on key teacher training courses.
The government is due to give more details of its plans this autumn.
In opposition, the Conservatives talked of the need to “raise the bar” in the teaching profession and attract the best graduates.
Mr Gove said England should look to countries such as Finland and South Korea, where only top graduates can become teachers.
The Conservatives’ education manifesto said they would only pay for graduates to train as teachers if they had at least a second-class degree.
But a study of current entrants to the profession by Professor Alan Smithers and Dr Pamela Robinson of Buckingham University says that policy could lead to “big holes” in teacher training courses for maths, physics, chemistry and languages.
A quarter of those who trained as physics teachers in 2008-09 would not have made the grade, along with 20% of maths recruits.
Professor Buckingham says while the aim of improving teacher quality is a good one, a blanket bar on third class degrees is not the way to achieve it.
“The idea is rather formulaic,” he told BBC News.
“We need to have high quality teachers and that is the key to improving our education system and making schools better, but it is a matter of organising it in the right way.”
The study says: “Finding teachers for key subjects will become even more difficult if the government will not fund the training of those with poor degrees”.
“Poor teachers are bad news for pupils, but is it better for physics to be taught by a well-qualified biologist than someone who has studied the subject at university even without much success?
“Michael Gove is putting the cart before the horse. Improving quality depends on attracting sufficient applicants to be able to choose those who can make subjects come alive for children.”
The proposals would not make much of an impact on the recruitment for trainee teachers in subjects such as history and English, where courses are heavily over-subscribed.
According to the report, 78% of graduates training to be history teachers in 2008-09 had a 2:1 or a first. For English, the figure was 73%, while for maths it was 47%.
Across all subjects in that year, nine out of 10 graduate trainee teachers did have a second- or first-class degrees, according to this study.
Just under 60% had a first or a 2:1, while about 10% had thirds or below.
This is similar to the UK-wide picture for graduates in general.
In 2008-9, a total of 62% of students getting their first degree achieved a 2:1 or a first.
Another 30% achieved a 2:2, while just under 8% were awarded thirds, data from the Higher Education Statistics Agency shows.
The coalition government says its top priority for the schools workforce is to improve the quality of teachers and that ministers are looking at a “range of plans”.
More details will be set out in a White Paper before Christmas, officials say.
A spokesman for the Department for Education said: “The countries which give their children the best education in the world are those which value their teachers most highly and where the profession attracts the brightest graduates.
“Our priority is to deliver robust standards and high quality teaching to all, whatever their background. To do this we must attract highly talented people into education, because the quality of teachers has such a huge influence on children’s achievement.
“The department is currently working to identify the most effective ways of doing this and further details will be made available in due course.”
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