Pupils may be prevented from staying on at their schools post-16 because of the introduction of reformed A-levels, new research suggests.

Nearly a third of sixth-form heads plan to toughen their admission requirements when the new linear A-level courses come in next year, according to a poll by the AQA exam board.

Heads’ leaders are warning that the reforms ordered by education secretary Michael Gove to encourage a “revival of the art of deep thought” could instead curtail young people’s ambitions.

Brian Lightman, Association of School and College Leaders’ general secretary, said: “We are concerned that this could close off opportunities for some students if there is not an adequate range of alternative [qualifications] that have equal status.”

The exam board’s survey reveals that the majority of schools and teachers are still unprepared for the new A-levels, which will start to be introduced from 2015.

It also suggests that nearly a third of sixth-form heads expect to students to choose subjects where A-levels have yet to be reformed, in a bid to escape the new linear courses.  

The reformed qualifications will replace existing modular A-levels and will prevent results from AS-levels - that were taken halfway through two year courses - contributing to overall A-level grades.

Oxford and Cambridge universities are both concerned that the move will make it harder for students to demonstrate their potential.

Mr Lightman said that he thought sixth forms were planning to increase entry requirements because the A-level stakes had been raised. The new arrangements could mean students “spending two years attempting a very demanding A-level and then failing it”. “That would be a waste,” he said.

Mr Gove hopes that the linear structure of his new A-levels will encourage “deep thought”. But the qualifications’ academic standards are supposed to be set at exactly the same level of demand as the old ones.

However, Mr Lightman said: “It is absolutely certain that the new A-levels are going to be very challenging indeed and while schools will do everything to prepare students for these more difficult exams these changes do risk some students not being able to access those advanced courses.”

The survey findings came from a session hosted by AQA at the Ucas annual conference earlier this month, attended by more than 80 teachers who the exam board said were “mostly heads of sixth form”.

When asked: “Do you expect to increase your admission requirements in light of the move to linear reformed A-levels?” 73 responded and 22 - or 30 per cent - answered “yes”.

The survey also revealed that only a fifth said they felt confident about their school’s introduction of the new A-levels next year. Just 17 per cent said their teachers were prepared for teaching the linear courses.

Mr Lightman is not surprised. “We have had concerns all along about the timescale for implementation of these changes... particularly bearing in mind that the first students taking these courses will not have had experience of linear examinations,” he said.

“It is a massive challenge for schools and we do have serious concerns about the way it’s being rushed.”

AQA’s head of general qualifications business support, Marc Booker, said: “We are putting in place a comprehensive package of support to help teachers prepare to teach reformed GCSEs and A-levels, and have ensured that the Department for Education (DfE) and Ofqual understand the importance of schools having a full year to do this.”

Earlier this month the DfE was forced to delay the introduction of the geography A-level a year until 2016, on the advice of academics. 

The AQA survey found that 30 per cent of the teachers said they expected “an increase in the number of students choosing unreformed modular A-levels” when the linear courses came in.

Students who do take that option would be left with very limited choices as all the main subjects apart from languages, maths, further maths, and now geography - will be reformed from 2015.