Clever students predicted to score an A and two Bs but who drop a grade and get three Bs in their A-levels could find themselves forced out of university places.
Changes to the admission systems at universities could mean bright teenagers who drop a grade in their exams miss out on a coveted degree place, according to a school leader.
Brian Lightman, general secretary of the Association of School and College Leaders (ASCL) also suggested school leavers are now considering alternative options to university, such as apprenticeships and on-the-job training - following the hike in tuition fees.
His comments come the day before around 300,000 students across England, Wales and Northern Ireland are due to receive their A-level grades, and learn whether they have won a place at university.
Last year, the proportion of A-levels scoring at least an A grade fell for the first time in more than 20 years.
Official figures for 2012 showed that 26.6% of the exams were given an A or A*, down from 27% in 2011 - a record drop of 0.4%.
Around one in 12 (7.9%) exams were given an A* grade, down from 8.2% in 2011, while 76.6% of entries got at least a C grade, up from 76.2% the year before.
Under a major overhaul of higher education in England, this year there is no limit on the number of students with an A grade and two Bs at A-level that universities can recruit, effectively leaving them competing to attract the best candidates. A number of leading institutions are expected to offer last-minute places to sixth-formers who do better than they expected and are above the cap.
It is thought that around 115,000 bright students will fall into this category.
But it also means that universities are likely to have little flexibility for students who fall just below the cap, and are therefore subject to strict number controls.
Asked about students who score three B grades, but had been offered university places based on higher results, Mr Lightman said: "They could well be squeezed out, and they might be very good students. There's a very, very small gap between one grade and another, it could just be a slip of the pen as it were. B grades are still very good grades."
Universities will "not be able to go above" their student number controls, he said, adding "that's very difficult for them to do".
Institutions that do exceed their allocated numbers risk facing a fine.
"I would say to those students (who miss out) step back and remember that you have still done really well," Mr Lightman said. "You have still got really good qualifications that will give you lots of opportunities. And take heart, because you are still very well qualified and well placed to be successful in life."
He also said that more work-related and vocational training was being made available to young people as an alternative to university.
"Firstly there's almost a growing backlash because of the inference, certainly from government, of A-levels as preparation for university to the exclusion of all else."
"For some students the decision will be do I go to university or do I look at one of the many other employment routes? There are an increasing number of high level routes available," Mr Lightman said.
"If you're paying £9,000 a year for university, people are going to ask about what they're getting out of it."
In general, many leading universities ask for at least an A and two B grades, although some courses will be available for students with A-level results below this.
Dr Wendy Piatt, director general of the Russell Group, which represents 24 top institutions including Oxford and Cambridge said: "We hope this year's change to a lower threshold of ABB or equivalent will reduce some of the unintended consequences from last year when students who wanted to attend a leading university and had the right qualifications were not able to - even when those universities wanted to accept them.
"One consequence of the uncertainties in the new system is that universities may have more places to offer through clearing to well-qualified students who have narrowly missed out on their first choice or through the adjustment process for those who have done better than expected."
UCAS, the admissions service said that according to its latest figures, applications have risen by 3.1%, with more students holding offers and more people already securing a place compared to the same time last year.
One education expert has predicted that A-level results could rise slightly tomorrow as ''practically-minded'' students turn to studying vocational courses and apprenticeships.
Professor Alan Smithers, director of the Centre for Education and Employment Research at Buckingham University said it was difficult to say what the pass rates would be this year, but that it was possible that the A*-C results could go up by tenths of a per cent.
''The more practically-minded in the past may have been encouraged to take A-levels, and haven't done well. Now they're transferring to practical qualifications and that could affect the overall pass rate," he said.
It has been suggested by others that an increased focus on traditional subjects, such as maths and science, could fuel a slight drop, as youngsters who may not have considered taking these subjects in the past, and may not be as strong in them, are now opting for the courses to help their chances of securing a university place.
Paul Clark, director of policy at vice-chancellors' group Universities UK, said: "Universities are prepared, as ever, for the busy admissions period this week. Despite the annual predictions of chaos, universities are very experienced in this area and admissions departments will cope remarkably well.
"This year, we have seen more applications, more offers made and more applicants placed compared with this time last year. As will be the case for hundreds of thousands of applicants, if they get the grades asked for in their offer, they will get their place. For students who miss out narrowly, there may still be opportunities to find a course that suits them via clearing. Last year, over 50,000 accepted places via this route."
He added: "Demand for university courses remains very high, with more people applying than there are places.
"Higher education is still a good investment as employers tell us that they are going to need more people with graduate-level skills in the coming years. UK degree holders are in demand and continue to earn considerably more than non-graduates over a working lifetime.
"While some have suggested that there will be a growing focus on a small number of 'jobs-based' degrees, we must not forget the importance to the UK of having a broad range of subjects.
"Higher education, regardless of subject, also provides graduates with a range of important skills such as the ability to think critically and to analyse and present evidence."Suggest a correction