Teenagers were today waking up to their GCSE grades amid indications that some schools are seeing major changes to results.
Early reports suggested that there are particular concerns among some headteachers about English and maths grades.
England's exams regulator, Ofqual, has previously warned that there is likely to be "variability" in individual school results this summer due to the significant alterations to the qualifications.
A move to end-of-course exams, more students taking international GCSEs (IGCSE), cuts to re-sits, a toughening up of GCSE geography, and a decision by Government that only a youngster's first attempt at a GCSE will count in school league tables are all likely to affect this year's results, the regulator has said.
Further changes, such as speaking and listening assessments no longer counting towards a student's overall GCSE English grade and a move towards more exams and less coursework in the subject, could also have an impact.
Hundreds of thousands of pupils across England, Wales and Northern Ireland are receiving their grades.
Ahead of the results being published, Brian Lightman, general secretary of the Association of School and College Leaders (ASCL), which represents a large proportion of secondary heads, said: "We are getting some individual reports of volatility, but we don't know about overall trends yet.
"Some schools have seen surprises. Some schools have seen results which are lower than expected."
One headteacher reported a drop of 18 percentage points in the proportion of students achieving at least a grade C in English.
She told the Times Educational Supplement (TES): "We're absolutely furious. Our overall results are now below the floor target (of 50% being introduced next year), and it's all down to English. The kids are going to be distraught."
Another said they had seen a "significant drop" in English language A*-C pass rate, and a third told the TES that in maths, the majority of his school's students had achieved "at least one grade lower" than predicted.
One academic suggested that schools which relied heavily on "gaming" the system by entering pupils for exams early and multiple times could be in for a "sharp shock", with lower grades than in previous years.
Professor Alan Smithers, director of the Centre for Education and Employment at Buckingham University, said that in recent years schools had devised ways to help students struggling to reach a C grade in core subjects like English and maths, in part because they were a key part of government measures used to judge a school's performance.
This included putting pupils in for exams early to give them practice, which allowed them to identify where youngsters were falling short, he said, while some schools used to be "quite generous in assessing speaking and listening".
These methods were no longer possible following the changes to the exams.
"Individual schools are likely to be affected differently according to how much they relied on gaming the old system so for some there could be sharp shocks in store," Prof Smithers said.
Mr Lightman said teachers and students had worked hard to achieve their results, despite constant upheaval.
"The constant piecemeal changes to GCSE exams are making it increasingly difficult for schools to prepare for GCSE exams and to accurately predict what students will achieve," he said.
"Despite constantly shifting exam goal posts, teachers and students have worked incredibly hard to achieve the results that they will receive.
"If the Government wants to do its part to help young people get the education they need to succeed in life, it must stop this obsession with tinkering with exams."
Nansi Ellis, assistant general secretary for policy at the Association of Teachers and Lecturers (ATL), said: "Continually changing the format of assessments with untested tinkering has to stop.
"It is not allowing young people to show what they can achieve, nor is it providing them with the skills they need for work and life after school or college."
In a letter published in June, Ofqual set out for schools and colleges the possible impact that exam reforms would have on this year's results.
"Collectively, these changes in the student mix are likely to result in a little more variability than usual, school by school," it said.
"When qualifications change, we would expect individual school results to be more variable, because the changes will have different impacts in different schools and in different subjects.
"It is not possible to predict at this stage how the national picture will look: these changes do not pull results universally in one direction or another, but together they are likely to affect the national picture to some extent."
Last year, the proportion of GCSEs awarded at least a C grade fell for the second year running.
Just over two-thirds (68.1%) of entries scored A*-C, down 1.3 percentage points from 2012 - the biggest fall in the exam's 25-year history.
The proportion of entries gaining top grades also fell by 0.5 percentage points - with 6.8% achieving an A*.