GCSE and A-level students are being told their grades will be protected from Covid disruption, as exams get under way for most students in England, Wales and Northern Ireland.
Extra measures are in place to help pupils, but support varies across the different parts of the UK.
Covid led to an increase in top grades in 2020 and 2021, with results based on teacher assessments instead of exams.
England's exam regulator called this year a "step back to normal".
Unlike last year, students in England have not been given advance information about the topics they are likely to be tested on. Grades are expected to fall back in line with results in 2019.
However, some of the adjusted measures from last year remain in place.
Exams will be spaced apart more than they were prior to the pandemic, allowing for rest and revision.
GCSE students will be given formulae and equations in some subjects, and will not be expected to confront unfamiliar words in language exams.
Outside England, grades are expected to remain higher than they were in 2019.
Teenagers in Scotland have already started exams. Modifications introduced in
Covid, such as paring back some assessments, remain in place.
In Wales and Northern Ireland, many students have been given advance information about what will appear in their exam papers.
Vocational technical qualifications, such as BTec courses, are still mostly assessed through practical learning but some exams and assessments have taken place across the year.
As pupils head on study leave, exam boards have warned students not to approach social media accounts claiming to sell exam papers.
In most cases, the papers for sale are fake. Nonetheless, BBC News has been told some students are paying up to £4,000 to get hold of them.
Margaret Farragher, chief executive officer of JCQ, said some accounts on TikTok, Instagram, Snapchat and other platforms "seem intent on scamming students for money".
"This issue is increasing every year," she said.