Heads fear clearing confusion over university applications
There are fears that the annual rush for last-minute university places will be more fraught than usual this year.
Students and their parents and teachers could face extra stress due to changes in the system, head teachers' leaders are warning.
For the first time, universities can take as many top-performing students as they can accommodate, provided they have A-level grades of at least AAB.
Heads say some pupils may be left in limbo.
The Association of School and College Leaders says it is worried that students who narrowly fail to get the grades they need to go to their first choice university may be left hanging on before that university decides whether to take them anyway - or reject them.
'Stress and worry'
Malcolm Trobe, ASCL's deputy general secretary said: "In the past they might have been taken if the university was under its quota [of students it is allowed to take], but it's possible those youngsters will be put on hold while the university goes fishing for others.
"In the past universities took them or rejected them immediately in which case they knew where they stood and could go in to Clearing immediately.
"The worry is that this will put a lot of youngsters in a state of limbo and uncertainty, bringing stress and worry for them and their parents, teachers and lecturers."
Clearing is the system which matches university applicants without courses to vacancies. About 50,000 people go through Clearing each year.
Pupils in England, Wales and Northern Ireland will get the results of their A-levels next week on Thursday and Clearing opens formally that evening, meaning that students can then apply for other courses.
Teenagers in Scotland have already got the results of their Highers and Advanced Highers.
About one in three of England's 18-year-olds has applied for a university place.
This is the first time that England's universities have been told they can take extra students above the quota allocated to them, on condition that they have top grades.
They have what is known as a "core" of places and if they go over that number they are fined per student.
Universities or colleges which charge less than £7,500 a year in tuition fees are also allowed to take extra students above their "core", up to a national limit of 20,000.
The changes are aimed at injecting market forces in to the system when higher tuition fees come in for England this autumn and at allowing good universities to expand.
The government had hoped there would be a variety of fees charged, but two-thirds of universities have opted to charge the maximum fee of £9,000 a year for some or all of their courses.
Ucas - the body which manages university applications across the UK - says it is confident that university admissions officers will confirm decisions "as quickly as possible".
A spokesman said: "The important thing to remember is that anyone who meets the conditions of the offer they have been given, getting the grades needed, will be guaranteed their place as in other years.
"If students have missed the grades in their offer, it is up to the university to decide whether they still want to accept them. They could use a range of factors to make their decision and so it may be a good idea for applicants to give them a call once they have their exam results to discuss their situation and see what their options are.
"Applicants with good grades will always be able to find vacancies in the system and reapplying next year is an option for those with their heart set on a particular course."
Head teachers' concerns have also been mirrored by some of those responsible for admissions at universities.
A body known as Spa (Supporting Professional Admissions) held surgeries for admissions staff in June and sent a report on them to universities.
This said that there was a strong feeling that more universities would be in Clearing, with some just looking for the extra people they could take on above their quotas and that there was a high risk that some confirmation decisions could be delayed, "resulting in greater uncertainty".
Exam results helpline
Ucas runs an Exam Results Helpline on 0808 100 8000, which has been open since 7 August.
It says advisors can talk to students about a range of options, not just university, including re-marks, re-sits, subject choice, apprenticeships and other vocational options, moving out of home and setting up in business.
Universities will also be able to take students in the AAB scheme if they have a variation of those grades. A list of grade combinations and alternative qualifications which can be counted as AAB by universities has been updated.
It shows that universities will be able to take students over their quota if they have grades including A*BB or A*AC for example, as well as those with a score of 35 points in the International Baccalaureate.