About 750,000 pupils in England, Wales and Northern Ireland are finding out their GCSE results, with another year of record scores likely.
The proportion of entries awarded the top grades has risen every year since the exams were first taken in 1988.
For the first time, most courses this year were taken in bite-size chunks, but course work has been cut down.
The results come amid concerns that pressure to do specific, more academic subjects will "demotivate" students.
Schools will be able to give results to students from 0600, with one exam board, Edexcel, allowing pupils who have signed up beforehand to log in to a website and access them from home.
Last year saw 22.6% of exams awarded an A* or an A grade, up from 21.6% the previous year - while 7.5% gained an A*.
Education expert Prof Alan Smithers, of Buckingham University, has predicted that the figures may top 23% and 8% respectively this year.
'Culture of re-sits'
He said the shift to modular assessment, under which students can resit chunks of the course if they fail the first exam, might "push the results up".
Click to play
The BBC's Chris Buckler met pupils at Crawshaw School, near Leeds, as they got their results
But on the other hand, course work has largely been replaced with controlled assessments taken in test conditions, which might "hold them down", he added.
Previously, some young people "clocked up a lot of points by producing massive bits of work and perhaps drawing on the help of parents and friends", he said.
The change might also help narrow girls' lead over boys in the results, as female students tend to be better able to apply themselves consistently over extended periods of time, he said.
Modular GCSEs in a wide range of subjects, apart from English, maths, ICT and science, were taught from 2009.
But Education Secretary Michael Gove has announced they will now be phased out, in order to end the "culture of re-sits" in which pupils can resit modules as they go.
Students starting GCSEs from September 2012 will have to sit all their exams at the end of the course.
Subject 'pressure'
Most students receiving their results this year will have started their courses before Mr Gove announced the English Baccalaureate in 2010.
This refers to a set of A*-C passes in maths, English, a language, two science qualifications and either history or geography.
But the head of the Association of Teachers and Lecturers, Mary Bousted, suggested some pupils had been pressured to change to EBacc subjects during the past year.
"We hope that those pupils, whose schools felt pressured into changing their GCSE subjects midway to meet the English Baccalaureate, do not suffer as a result," she said.
GCSE grades over time
The new focus on EBacc subjects "risks demotivating and alienating" thousands of young people who struggle with academic subjects, she said.
Russell Hobby, General Secretary of the National Association of Head Teachers, cautioned against the "usual debate about what constitutes a good or bad subject".
"That misses the point... a well-taught media studies course is better than a badly taught English literature course," he said.
'Academic core'
A spokesman for the Department for Education said that the proportion of pupils doing these five core academic subjects has plummeted from half of all pupils in the late 90s to just one in five now.
The EBacc is not compulsory and "is only one measure of success", the DfE said, and "pupils should study what is right for them".
But the subjects formed an important core of academic study and would make young people more employable, the spokesman added.
The results come amid concerns about cuts to careers services and Education Maintenance Allowance study grants for low-income 16 to 19-year-olds.
On Tuesday, the head of the Association of School and College Leaders, Brian Lightman, warned that 16-year-olds faced "unprecedented challenges".
The numbers staying on in education between 16 and 18 are rising.
But DfE figures published on Wednesday showed a rise in the number of 18-24 year olds not in education, employment or training in England.
At 18.4%, the number is higher than any second quarter figure since 2006.
Most pupils in Scotland take Scottish Standard Grade and Higher qualifications, rather than GCSEs and A-levels.
They received their results in early August.
The proportion of entries awarded the top grades has risen every year since the exams were first taken in 1988.
For the first time, most courses this year were taken in bite-size chunks, but course work has been cut down.
The results come amid concerns that pressure to do specific, more academic subjects will "demotivate" students.
Schools will be able to give results to students from 0600, with one exam board, Edexcel, allowing pupils who have signed up beforehand to log in to a website and access them from home.
Last year saw 22.6% of exams awarded an A* or an A grade, up from 21.6% the previous year - while 7.5% gained an A*.
Education expert Prof Alan Smithers, of Buckingham University, has predicted that the figures may top 23% and 8% respectively this year.
'Culture of re-sits'
He said the shift to modular assessment, under which students can resit chunks of the course if they fail the first exam, might "push the results up".
Click to play
The BBC's Chris Buckler met pupils at Crawshaw School, near Leeds, as they got their results
But on the other hand, course work has largely been replaced with controlled assessments taken in test conditions, which might "hold them down", he added.
Previously, some young people "clocked up a lot of points by producing massive bits of work and perhaps drawing on the help of parents and friends", he said.
The change might also help narrow girls' lead over boys in the results, as female students tend to be better able to apply themselves consistently over extended periods of time, he said.
Modular GCSEs in a wide range of subjects, apart from English, maths, ICT and science, were taught from 2009.
But Education Secretary Michael Gove has announced they will now be phased out, in order to end the "culture of re-sits" in which pupils can resit modules as they go.
Students starting GCSEs from September 2012 will have to sit all their exams at the end of the course.
Subject 'pressure'
Most students receiving their results this year will have started their courses before Mr Gove announced the English Baccalaureate in 2010.
This refers to a set of A*-C passes in maths, English, a language, two science qualifications and either history or geography.
But the head of the Association of Teachers and Lecturers, Mary Bousted, suggested some pupils had been pressured to change to EBacc subjects during the past year.
"We hope that those pupils, whose schools felt pressured into changing their GCSE subjects midway to meet the English Baccalaureate, do not suffer as a result," she said.
GCSE grades over time
The new focus on EBacc subjects "risks demotivating and alienating" thousands of young people who struggle with academic subjects, she said.
Russell Hobby, General Secretary of the National Association of Head Teachers, cautioned against the "usual debate about what constitutes a good or bad subject".
"That misses the point... a well-taught media studies course is better than a badly taught English literature course," he said.
'Academic core'
A spokesman for the Department for Education said that the proportion of pupils doing these five core academic subjects has plummeted from half of all pupils in the late 90s to just one in five now.
The EBacc is not compulsory and "is only one measure of success", the DfE said, and "pupils should study what is right for them".
But the subjects formed an important core of academic study and would make young people more employable, the spokesman added.
The results come amid concerns about cuts to careers services and Education Maintenance Allowance study grants for low-income 16 to 19-year-olds.
On Tuesday, the head of the Association of School and College Leaders, Brian Lightman, warned that 16-year-olds faced "unprecedented challenges".
The numbers staying on in education between 16 and 18 are rising.
But DfE figures published on Wednesday showed a rise in the number of 18-24 year olds not in education, employment or training in England.
At 18.4%, the number is higher than any second quarter figure since 2006.
Most pupils in Scotland take Scottish Standard Grade and Higher qualifications, rather than GCSEs and A-levels.
They received their results in early August.
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