Sixth-formers are facing yet more exam pressure under a new system of university admissions to be introduced next summer.
Tutors are to be given the power to make offers conditional on students' performance in individual A-level papers, because of the difficulty of having to choose between candidates with multiple A grades.
Some schools and colleges believe that universities have no choice but to introduce the measure, given that Oxford and Cambridge now reject about 10,000 students a year with three As.
But it is still causing serious concern. A trial of the plan will take place this summer, with 11 universities told how well students did in each of their AS and A2 modules. They will not, however, be able to select students on the basis of this extra information.
From next year, they will be able to specify that students must achieve a certain number of good grades, and even to demand a minimum grade for a specific module, known as a unit.
John Tredwell, principal of Worcester sixth-form college, said:
"Potentially this is going to be a bit hard on a student who has done very well on five units, but for some reason underperforms on a sixth. This is going to put quite a lot of pressure on students."
John Guy, principal of Farnborough sixth-form college, Hampshire, questioned whether marking was reliable enough for decisions to be made on the basis of single papers. He cited the case of one former student, now at Oxford reading English, who achieved grade As in five of her six A-level philosophy modules, but a D for her extended essay.
Dr Guy said the student was one of the brightest anyone at the college could remember, and that the essay was probably the most sophisticated he had ever read from an 18-year-old. Under the new system, the student might have missed out on Oxbridge if As in each module were required.
Geoff Lucas, secretary of the Headmasters' and Headmistresses' Conference, said the move could become a "nightmare" for students if different universities required high grades in different modules.
But some heads have welcomed the move as better than one alternative: universities setting their own tests to choose between students.
Ian MacNaughton, principal of Colchester sixth-form college, said: "I do not have a problem with universities getting individual module grades. If the information is available, people should have access to it."
A spokesman for Ucas, the admissions co-ordinator, said: "Ucas is here to open up and make higher education available to as wide a group of people as possible. We do not seek to disadvantage anyone through that process.
"We are still testing possible ramifications of how unit-grade information will be used taking into account the impact on applicants, universities and colleges and, indeed, government policy."
Sixth-form pupil Hannah Caswell, 18, of Kidderminster, hopes to study law at Nottingham or Manchester. She said: "Something does need to be done, because there's not much to differentiate between three As for people going to university, but I'm not sure if this is the right answer."