The remodelled A-levels introduced this year could trigger a repeat of the grading scandal that engulfed Government and the entire exams system in 2002, independent school heads fear.
The TES has learnt that concerns are so acute that heads will sit in on crucial exam board grading meetings this summer, a measure that has not taken place since the immediate aftermath of the debacle eight years ago.
That crisis had enormous repercussions: it was one of the reasons for the resignation of education secretary Estelle Morris, led to the sacking of Qualifications and Curriculum Authority (QCA) chairman Sir William Stubbs, and caused a major loss of confidence when it resulted in the work of 10,000 pupils being re-graded.
Then, as now, the introduction of a new kind of modular A-level and AS- level coincided with many pupils receiving grades far lower than their teachers predicted.
And then, as now, it was the independent sector raising the alarm.
Geoff Lucas, Headmasters' and Headmistresses' Conference (HMC) secretary, writes in The TES today: "The grading of this summer's AS exams in some subjects led some who experienced the summer of 2002 to feel an uncanny sense of deja vu. We await this summer's A-level awards with some trepidation."
He said any change to exams crecarried risk, and this year's, which also include the new A* grade, formed a "potentially potent cocktail".
Andrew Grant, HMC chairman, fears there could again be a temptation to intervene with grade boundaries to ensure standards remain consistent with previous years, particularly as the new AS-level grades already appear to be tougher than their predecessors.
"With the new A2 (final A-level exam) awards we have got the possibility for that effect to be doubled up," the head of St Albans School and one of the 2002 whistleblowers said. "If the AS grades are already out of line with previous standards how are they going to correct that if they don't do something to manipulate the A2 grade?"
In 2002 it appeared that a desire to avoid a sudden sharp improvement in A-level results prompted crude last minute shifts of grade boundaries that led to some straight-A pupils receiving U grades.
Last summer saw the first results for new AS-levels, slimmed down from two to three modules, and again there were some surprising results. As The TES revealed in September, angry drama teachers were calling for "mass rebellion" against Edexcel. The exam board defended its performance, but pupils predicted high grades received Us.
The HMC also had concerns about other AS-levels offered by the OCR and AQA boards, including those in chemistry and English literature, which saw significant falls in the percentage of pupils achieving A grades.
Despite assurances from Ofqual, the exams watchdog, the HMC says it has yet to receive satisfactory explanations for the changes.
Members fear this summer's A2 grades, again involving a drop in modules, could compound the problems.
Representatives from the HMC, the Association of School and College Leaders and the Girls' Schools Association are expected to cover exam board meetings setting grade boundaries in all the main subjects to try and prevent another crisis.
A spokesman for the OCR board said heads had always been welcome at grade boundary meetings.
An Ofqual spokesperson said AS grades last summer were "fair and consistent" and the regulator was working with boards to ensure the same was true for A-levels.