Some pupils will be awarded the new A-level A* grade this summer despite achieving lower overall marks than classmates who only receive conventional A grades, schools are being warned today.
The seeming anomaly will occur because the A* grade is based mainly on the performance of pupils in the second year A2 exams, designed to be more challenging than the AS-levels they take in their first A-level year.
Heads' leaders predicted that the situation would be difficult to explain to parents.
Candidates will still have to perform well in AS-levels to stand any chance of gaining an A* grade.
To be deemed good enough to achieve the new award they must first reach the standards necessary to get normal A grades, awarded according to aggregate AS and A2 scores.
It is the second hurdle of having to get 90 per cent or more of marks in A2 exams that could cause the controversy for the A*.
In an open letter to secondary schools and colleges, published today, Kathleen Tattersall, chair of exams regulator Ofqual, writes: "It means that there will be candidates in summer 2010 who do not achieve an A* grade but who have higher A-level scores (which arise from the total units, both AS and A2) than candidates who have achieved the new grade."
An Ofqual spokesman said emphasis was placed on A2 because the new grade was designed to signify the highest-performing candidates and the A2 contained the most challenging questions.
John Dunford, general secretary of the Association of School and College Leaders, said: "I think this might cause some misunderstanding among parents and will be difficult to explain. But headteachers are clear that A* is only earned on the A2 papers."
The letter stresses that the percentage of A* grades is likely to vary from subject to subject in the same way as the percentage of A grades.
It also seeks to reassure schools about the fairness and consistency of A-levels this summer, the first to be awarded since they were slimmed down from six to four modules.
"Should any very unexpected outcomes occur, we will require an explanation and may ask the awarding organisation to take action to ensure that standards are maintained from 2009 and the awards are fair and consistent across the awarding bodies," Ms Tattersall writes.
Last month The TES revealed that top independent school heads feared the change to new style A-levels could trigger a repeat of the grading scandal that engulfed the Government and the entire exams system in 2002.
Pupil X achieves a B at AS-level, with 145 marks, and then does very well at A2, achieving 180 marks, totalling 325 marks overall.
Pupil Y achieves an A grade AS-level with 190 marks and gets 140 at A2 level, with 330 marks overall. Both pass the 320 mark threshold for an A grade in that particular A-level.
But, despite receiving fewer overall marks, only pupil X meets the requirement for an A* grade by achieving 90 per cent or more in the A2. Pupil Y does not.