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."
Top grades are up - and more teenagers than ever have won places at University. Tens of thousands of students received their A-level results today - and overall, there is the first slight dip in pass rates - the first time that's happened in more than 30 years. Right across the South there have been tears of joy and disappointment as pupils opened their envelopes. Many say it's been tougher than ever to gain good grades because of government reforms to the exam system. Our Social Affairs Correspondent Christine Alsford reports.
Two video reports: After weeks of anxious waiting, thousands of A-level students across the South East have been getting their results today.
Harry Dawes says he is "over the moon" after being accepted into a university in Kent.
He will study business studies with human resources management at Canterbury Christ Church University.
Think you can top that results day selfie? Tweet us @itvmeridian or find us on Facebook.
Triplets in Sussex have been celebrating their AS-level results this morning. The siblings got 8 As, 3 Bs and 1 C amongst them.
Students in Kent have been celebrating with results day selfies. Jordan Cooper, Hannah Clare and George Feas have all got the results they needed to do their courses at Canterbury Christ Church University.
Students at Varndean College in Brighton open their envelopes. Video also contains interviews with students.
Students across the region will get their A-Level results this morning but this year could see greater variation in results following a reform to the exams. Students are being told not to panic if they don't get the results they want.