Hundreds of thousands of teenagers in England, Wales and Northern Ireland are awaiting their A-level results on which the university places of many depend.
Teachers are warning of “volatility” in the results after changes to courses.
However, hundreds of pupils already know if they have been successful after a computer error leaked the results of their university applications.
There are expected to be more university places available this year, with suggestions of a “buyer’s market”.
The long wait for A-level and AS results will soon be ended, with pupils going into schools to get results or finding out online.
But a small number of universities accidentally sent out acceptances or rejections a day early, with the admissions service blaming automated computer systems.
Prof Alan Smithers, director of the Centre for Education and Employment Research at Buckingham University, has forecast that overall results will be similar to last year.
In 2015, there was a slight fall in A* and A grades awarded, down to 25.9% of entries. But the proportion of passes rose by 0.1 percentage points to 98.1%.
Teachers and head teachers’ leaders have warned that, below the surface of this stability, individual schools and pupils could face more unpredictable outcomes.
“Students and their teachers are bracing themselves for greater volatility in this year’s results,” said Ms Keates, leader of the NASUWT teachers’ union.
She said that schools had to contend with new content in AS and A-levels.
Head teachers’ leader Russell Hobby forecast “significant variation” and unexpected results, behind the picture given by the national statistics.
“For individual students, teachers and schools this is a real concern,” said Mr Hobby.
For those students who miss out on required grades for university, there are expected to be more opportunities available through the clearing system.
This will include leading universities within the Russell Group, which have signalled that they expect to have places on offer.
There is a demographic dip in the number of 18-year-olds and the removal of any limit on student numbers means that universities could be competing to attract students to fill places.
London South Bank University is going to run a 36-hours non-stop clearing service, remaining open all night for inquiries from students.
But students starting at England’s universities in the autumn could face higher fees than the previous cap of £9,000 before they finish.
Exeter University has announced that it is going to increase fees to £9,250 for current students and those beginning this year, as well as those starting in 2017.
There will also be opportunities for those not going to university, with financial services firm PwC announcing that it has had a 20% increase in two years in recruiting directly from those leaving school with A-levels.
A report from Barclays and the Centre for Economics and Business Research has argued that those entering apprenticeships after school could have much higher lifetime earnings than those going into graduate careers.
And a study from the Social Market Foundation, also released on results day, shows that vocational qualifications are an increasingly common route into university, often in combination with A-levels.
A Department for Education spokeswoman said: “A-level results day is the culmination of months of hard work for students and teachers and everyone involved deserves credit.
“Our exam and curriculum reforms will build on those achievements and ensure pupils have the knowledge and skills that will help them secure a university place, apprenticeships or a great job.”