The new GCSE grading system – which includes an additional top grade that is harder to achieve – has "ratcheted up the pressure" on teenagers, headteachers have warned ahead of results day.

Schools have given students extensive support to help ease their stress and anxiety in the wake of tough new GCSEs in England– where A* to G grades have been replaced with a 9 to 1 scale.

As few as 200 students could achieve a clean sweep of 9s - the new highest grade - in the reformed GCSE exams this year, a study has suggested, leaving many students “disappointed” with grade 8s.

Tens of thousands of pupils will receive their exam results on Thursday – and students in England will receive new numerical grades in 20 reformed GCSEs – including science, history and the languages.

Last year, students were graded using the new numerical nine-point scale for English literature, English language and maths – the first GCSE subjects to be made more rigorous.

School leaders are worried about the impact that the reforms – which include more demanding content, less coursework, and exams at the end of two years – have had on students’ mental health.

Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said: "We are concerned that the new grading system for GCSEs ratchets up the pressure on young people another notch.

"It was already very hard to achieve the top grade of A* under the old system, and it is even harder to achieve the top grade of a 9 under the new system.

"Young people striving for those top grades may therefore feel disappointed if they do not achieve them, even though they have done exceptionally well in the grades they do achieve."

Under the new grading system, there is more differentiation between results - with three top grades - 7, 8 and 9 - compared to two A* and A under the alphabetical eight-point scale.

Professor Alan Smithers, director of the Centre for Education and Employment Research at the University of Buckingham, has predicted that a higher percentage of students will gain the two new top grades - 8s and 9s - than the proportion of A*s awarded under the old system.

But he said: "Parents, pupils and schools have increasingly tended to see straight top grades as the acme of success in GCSE.

“It looks as if many are going to be disappointed this year in spite of the actual improvement at the top. It will not, however, feel like it because a grade 8, does not seem as grand as the old A*”

Mary Bousted, joint general secretary of the National Education Union (NEU), warned: "We know that students who could have expected to receive an A* in 'legacy' GCSEs will be disappointed if they receive an 8 rather than the top grade of 9.

"This sense of under achievement could lead to them deciding that they aren't good enough in those subjects to continue them to A-level."

A survey of teachers earlier this month, from the NEU, found that nearly nine in 10 (89 per cent) believe changes to the GCSEs have made students more anxious and stressed.

Meanwhile, charity Childline held 1,298 counselling sessions with children and teenagers worried about their exam results and what to do next in 2017/18 - a 15 per cent rise compared to the previous year.

Just under a quarter of counselling sessions were delivered last August - the month when exam results for the first of the reformed A-levels and GCSE subjects were published.

Mr Barton added: "The new grading structure is another aspect of a very significant set of reforms to GCSEs which has put young people under a great deal of additional pressure.

"We are worried about the impact on the mental health and wellbeing of young people caused by these reforms and it is our view that such a substantial set of changes as this should have been introduced in a more managed and considered manner.”

"It is to the credit of schools that they have responded to this situation by providing their students with extensive pastoral support in order to alleviate stress and anxiety despite severe funding pressures,” he added.

A-level results 2018 – in pictures Show all 21 1 /21 A-level results 2018 – in pictures A-level results 2018 – in pictures Students celebrate their A Level results from Ark Academy in Wembley PA A-level results 2018 – in pictures Charlie Jenner, Nathan Sharp and Abhinav Boddu receive their A Level results at Norwich School PA A-level results 2018 – in pictures Students collect their A Level results at Brighton College PA A-level results 2018 – in pictures Students celebrate their A Level results from Ark Academy in Wembley PA A-level results 2018 – in pictures Twins Himmat (left) and Arjun Sohal, who both got 3 A's in Maths, Physics and Chemistry, and will both be going to Bath Universirty to study Chemical Engineering, celebrate their A Level results at Peter Symonds College, Winchester PA A-level results 2018 – in pictures Students celebrate their A Level results from Ark Academy in Wembley PA A-level results 2018 – in pictures Alex Cox receiving his A Level results at Mander Portman Woodward College, Edgbaston, Birmingham PA A-level results 2018 – in pictures Students react as they receive their 'A' level results at Stoke Newington School and Sixth Form in London Reuters A-level results 2018 – in pictures Moyle Oloko with her results at Brighton College in East Sussex PA A-level results 2018 – in pictures Grace El-Mokadem, Maddie Backhouse, Claudia Ballard and Sophie Plumber read their A Level results at Brighton College in East Sussex PA A-level results 2018 – in pictures Twins Emily and Molly Goldberg receive their A Level results at Merchant Taylor School in Crosby, Merseyside PA A-level results 2018 – in pictures A student reacts as she receives her 'A' level results at Stoke Newington School and Sixth Form in London REUTERS A-level results 2018 – in pictures Students react as they receive their 'A' level results at Stoke Newington School and Sixth Form in London Reuters A-level results 2018 – in pictures Simran Kaur Bahia 20 receiving her A Level results gaining an A*, A and B at Mander Portman Woodward College, Edgbaston PA A-level results 2018 – in pictures Students pose for a photograph as they receive their A Level results from from Merchant Taylor School in Crosby, Merseyside PA A-level results 2018 – in pictures Amanda Georgevic with daughter Phoebe Georgevic 19, who got two A's and one B in her A Level results at Mander Portman Woodward College, Edgbaston PA A-level results 2018 – in pictures Students react as they receive their 'A' level results at Stoke Newington School and Sixth Form in London Reuters A-level results 2018 – in pictures Harriet Selway, 17, left, and Niamh Davies, 17, right, both from Horfield, Bristol, react as they open their A Level results at St Mary Redcliffe and Temple School PA A-level results 2018 – in pictures Eleanor Elizabeth Sykes 18 receiving her A Level results gaining 2 A's at Mander Portman Woodward College, Edgbaston PA A-level results 2018 – in pictures A group of students talk after receiving their A Level results at Mander Portman Woodward College PA A-level results 2018 – in pictures A student reacts as she receives her 'A' level results at Stoke Newington School and Sixth Form in London Reuters

The reforms to GCSEs in England come as part of a government drive to ensure young people have the knowledge and skills needed to go on to work and further study.

The new grades – including the grade 9 – were introduced to help employers and universities to better differentiate between students of different abilities.

A Department for Education spokesman said: "Our new gold-standard GCSEs don't just raise academic standards but the new grade 9 provides stretch at the top end of the ability range to recognise and reward truly exceptional performance."

Ahead of GCSE results day, Professor Smithers has also predicted that boys will close the gap with girls at the top grades – as less coursework and more exams is supposed to favour boys.

Last week, boys outperformed girls at the top grades in A-level for the second year running. Overall, 26.6 per cent of entries for boys were awarded A* or A grades, compared to 26.2 per cent for girls.