Headteachers have expressed "grave concerns" over an unexpected and substantial fall in grades and the number of passes in this summer's revised AS-level exams.
In a damning letter seen by TES Cymru, the Cardiff Conference of Secondary Headteachers (CCSH) accused the WJEC exam board of bowing to political pressure and introducing a tougher marking system in order to "raise the bar" for A-levels.
The letter's author Tim Pratt, vice-chair of the CCSH, said that in some city schools up to 21 per cent fewer students achieved an AS-level pass grade this year.
Mr Pratt, who is also head of St Teilo's Church in Wales High School in Cardiff, said heads had also seen a considerable reduction in the number of their students gaining top A and B grades.
As a result, a "significant number" of students have dropped out of school, leading to lower sixth-form numbers and depleted budgets, he claimed.
In the letter to the WJEC, Mr Pratt wrote: "It is now too late to do anything for those students whose hopes and aspirations have been destroyed, but we would like to seek some reassurance that this sort of action will not reoccur in 2010."
Parents have also expressed anger. Pam Lake was shocked when her daughter Clarice Watkinson, a student at Whitchurch High School in Cardiff, received E grades in both her AS level English and French exams after predictions of Bs.
Ms Lake, a careers service manager at Cardiff University, said: "It is evident something has gone wrong, and I feel they (exam boards) have a duty to look outside the box and see what has happened. Our children's futures are at stake here."
The unexpectedly low results are not restricted to Cardiff. Although the CCSH wrote to the WJEC as the main awarding body in their schools, it is thought that the lower results apply across other exam boards too.
The new unit-based AS level specifications were introduced last year, and all awarding bodies in England and Wales were required to adopt a different awarding methodology.
In its response to the CCSH, the WJEC denied this amounted to political pressure, but admitted it was reviewing the 2009 AS-level outcomes in detail.
Derec Stockley, WJEC director of examinations and assessment, said: "These are new specifications with, in many cases, a different number of units and a different pattern of entry across January and June, which makes direct comparability with the previous year difficult.
"The 2009 cohort should be neither advantaged or disadvantaged by the fact that they are the first to be following the new specifications."
The Assembly government acknowledged there had been a dip in AS level results in Wales after "several particularly good years", but said it would be "unwise" to draw conclusions.
A spokesman said the regulator had so far found no "no indications of an overall leaning towards either leniency or greater difficulty".
"As a general rule, it tends to be true that, where examinations change, some students and schools will achieve results that are better than expected and some will be disappointed. Maintaining standards for exams is a matter for the regulatory bodies and is not subject to political pressure."