Thousands of students in North Devon will joining more than 600,000 teenagers in England, Wales and Northern Ireland by getting their GCSE results today.
Last year saw the first fall in the exam's history in the proportion of entries awarded top grades.
England's exams watchdog, Ofqual, has warned that science grades could fall because papers were toughened up.
Teaching unions say grades will be hard to compare with other years.
Last year, just over one in five exam entries (22.4%) was awarded either an A or an A*.
As well as the changes to science, which apply to England and Wales only, marking and grading was also tightened for English and maths this year.
And for the first time this year, pupils in England, Wales and Northern Ireland will get extra marks for good grammar and spelling in key subjects.
A total of 5% of the marks in English literature, geography, religious education and history will be allocated for this. Students were always given a quota of marks for this in English language papers.
Any changes to results could vary around the UK as changes brought in do not apply across the board.
Because education is a devolved matter, Wales and Northern Ireland make their own decisions on exams. Scotland has its own system and its results were released two weeks ago.
Officials in Northern Ireland predict results there will be "quite stable".
Changes to the marking and grading of English GCSEs in England and Wales were brought in after an outcry over last year's GCSE English results which led to a legal challenge.
Ofqual recently said English, maths and all science results were likely to "look different", with grades possibly dropping, because of changes to the exams and the way in which some schools entered pupils early for exams or for multiple exams in the same subject.
It has criticised some schools for entering students for exams in the same subjects through different boards to try to get the best grades.
Ofqual's head Glenys Stacey has said the changes brought in will ensure standards are "maintained" and that students will get the "right results".
The Association of School and College Leaders (ASCL) - which represents secondary head teachers - says it is expecting "significant turbulence" in this year's results.