The biggest impact on this year's GCSE results have stemmed from changes that mean students did not sit exams early, compared to previous years when pupils could take GCSEs multiple times, exam chiefs suggested.
This year, only a teenager's first attempt at an exam would count in school league tables, so schools that had traditionally made use of the winter exam season, entered pupils early, or made use of resitting are likely to have seen the greatest changes.
There has been a significant amount of change to the system this year and although UK level figures are relatively stable we expect more schools and colleges to see volatility in their results. The extent of this volatility will depend on how much change from their usual practices they experienced and how they adapted.
Entry patterns are very different this year. We have seen a dramatic decline in the number of entries from 15-year-olds, which is largely due to a change in the school accountability measure, where a candidate's first entry counts in performance tables, and the move to end-of-year exams in England.
As we would expect, where the change in entry patterns is greatest, such as the sciences, English and maths, we have seen some impact on results. But despite these changes and the potential for increased centre volatility, candidates can be confident that standards have been maintained.
Today's GCSE results show that girls once again lead pass rates at grade C and above, with 73.1% of girls' entries scoring A*-C compared with 64.3% of boys'.
However, official results showed that boys are beginning to close the gap at A*, with 5.2% of entries scoring the top grade compared with 8.1% of girls'. The difference of 2.9 percentage points is down from three percentage points last summer.
The proportion of GCSE exam students awarded at least a C grade has risen for the first time in three years, official figures show.
Just over 68.8% of exam entries scored A*-C - up 0.7 percentage points on last summer, statistics published by the Joint Council for Qualifications (JCQ) show.
In maths, 62.4% of students were awarded an A*-C grade - a significant 4.8 percentage points on last year's results.
Exam chiefs suggested that changes to this year's entries, including fewer lower-performing 15-year-olds taking the GCSE early, are behind the hike.
In contrast, 61.7% of English entries scored a C or higher, down 1.9 percentage points from last summer.
The drop - believed to be the biggest in the qualification's history - could be down to strong candidates taking advantage of the chance to sit the exam last winter, the JCQ suggested.
The proportion of entries awarded an A* - the highest grade - has dropped to 6.7% from 6.8% last year. It is the third year in a row that the number of students achieving the top pass rate has fallen.
Schools preparing to receive their GCSE results today have been told to expect "variable" grades.
There are particular concerns among some headteachers about English and maths grades, according to initial reports.
The potentially unpredictable results are said to be due to significant alterations to the qualifications this year.
Brian Lightman, general secretary of the Association of School and College Leaders, 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."
Education Secretary Michael Gove said he wanted "schools to be able to nurture creative talent in every child" after announcing that exams in arts subjects are set to become tougher.
I am passionate about great art, drama, dance, music and design, and I am determined to ensure every child enjoys access to the best in our culture. I also want all schools to be able to nurture creative talent in every child.
That is why I am delighted that new high-quality qualifications in creative and cultural subjects will be made available to all students.
They will now have the chance to take these new qualifications from September 2016.
A major reform of the exams system will see GCSEs and A-levels in arts subjects such as music and drama undergo a radical overhaul in a bid to make the courses tougher, it has been announced.
A total of nine GCSEs and six A-levels will be reformed under the Government's plans, in a move that ministers say will give pupils in England access to "high-quality" qualifications in creative subjects.
The Department of Education said that from September 2016 the following new GCSEs will be available in schools; art and design, music, drama, dance, citizenship, computer science, design and technology, PE and religious studies
They will be taught alongside new GCSEs in history, the sciences, geography and foreign languages - the subjects contained in the Government's English Baccalaureate (EBacc).
Groups promoting the arts, design and religious education previously expressed criticism that too much attention was put on exams in traditional subjects and creative disciplines had been left out.
The Association of School and College Leaders General Secretary Brian Lightman is due to call on the government to respond unanswered questions over reforms to qualifications such as the GCSEs, after warning that students and teachers faced another three years of turmoil.
ASCL Curriculum and Qualifications Specialist Sue Kirkham, who is leading a seminar on the reforms at the conference today in Birmingham, said:
We don't yet know what format these examinations will take, what the precise content of the syllabus will be in most subjects or what kinds of questions will be asked.
All of this needs to be known before teachers and school leaders can plan effectively in order to prepare from them. Also, awarding bodies need to be trusted to brief teachers ahead of the changes so that teachers can begin to prepare.
Poor planning and delay over changes to qualifications and other reforms will lead to three years of turmoil for students and teachers, the Association of School and College Leaders will warn at its conference today in Birmingham.
The association has today published a list of seven issues which have yet to be decided by the Department for Education and education body Ofqual, which are hindering teachers' ability to plan and prepare students for the new GCSEs, which should start to be taught as early as September 2015.
Hundreds of GCSE, AS and A-level results have been re-graded after errors were made in exam marking.
The OCR exam board said it apologised "unreservedly" for the mistakes, which were made by examiners, and insisted that action had been taken.
In total, 98 GCSE, 285 AS-level and 50 A-level results, were revised upwards after being re-marked.
The errors came to light in the autumn and were due to "human and process errors by a minority of examiners", the board said.
Schools and colleges with affected students have been told, OCR said, and they will pass on the results to their pupils.
Chief regulator Glenys Stacey says Ofqal has noticed a pattern in appeals of exam results from schools.
The analysis that we have done has shown that there is tactical appealing at critical grade boundaries - C/D at GCSE and B/A at A-level, that doesn't mean that every appeal at that boundary is tactical, but you can see from the pattern that it would suggest there is tactical appealing.
Secondly, the way the appeal system is designed at the moment, and indeed headteachers agree with us, that where you're within a couple of marks of such a grade boundary it's worth appealing because it is a one-way bet.