When is a GCSE not a GCSE?" It may sound like a riddle from a teacher's Christmas cracker but, for an increasing number of schools in the UK, this is no joke.
The answer, of course, is "when it is an IGCSE" (or International GCSE to give it its full title). Offered by both Cambridge International Examinations (CIE) and Edexcel in some 60 subjects, and taken by about 3,000 candidates in the UK (and more than 100,000 students worldwide), the IGCSE is not recognised or accepted into the national qualifications framework by the Qualifications and Curriculum Authority.
Consequently, it attracts neither points in the Government's examination performance tables nor funding in the maintained sector. This is in spite of the fact that universities recognise IGCSEs as equivalent to conventional GCSEs for admissions purposes.
Although only independent schools at home and abroad can offer the IGCSE, those that do so in this country risk jeopardising their league-table position. An extreme example, from a school in north-west England, illustrates this starkly.
With some 70 "home" students doing GCSEs and 50 from abroad at the school's international study centre doing IGCSEs, an 87 per cent grades A*-C success rate is reduced to 64 per cent in the official figures, simply because the IGCSE results are discounted.
While this may seem an isolated and extreme example, government proposals in its 14-19 white paper to report five grade A*-C GCSE achievement, including English and maths from 2006, turns this into a much wider concern - these are the subjects proving increasingly popular with schools switching to the IGCSE.
At the heart of the debate lie fundamental issues of choice, standards and the inclusiveness of the qualifications framework.
So far as choice is concerned, there is little doubt that many schools are turning to the IGCSE because, in subjects like maths, coursework is not compulsory. Until the last changes to the national criteria for GCSE maths some years ago, all schools could choose whether or not to do coursework.
That choice is now denied them.
Hopes that the Tomlinson 14-19 reforms would lead to a radical pruning of coursework, at GCSE at least, have now been dashed. And the flurry of recent interest in the IGCSE can be traced to this.
But its appeal goes beyond the jettisoning of coursework. Teachers with experience of both systems talk enthusiastically of the IGCSE's "less obtrusive" assessment style, which leaves more scope and time for teaching beyond the syllabus; a reduced overall assessment burden (even where coursework is included); and greater differentiation through questions which are better targeted, allowing candidates at both ends of the ability range to demonstrate what they know, understand and can do.
They are unanimous that the IGCSE is as good, if not better, as preparation for A-level.
Such perceptions inevitably open up the thorny issue of standards. While teachers are adamant that the IGCSE is every bit as demanding as GCSE - many claim it is more so - empirical evidence is limited.
However, the CIE undertakes regular comparability checks, and some Edexcel subjects share the same principal examiners. A comparative study of the exams, commissioned by the Headmasters' and Headmistresses' Conference (HMC) and the Girls' Schools Association, supports the anecdotal evidence from teachers that standards between GCSE and IGCSE are broadly comparable.
So far as end-users are concerned, this is certainly the case.
Both employers and universities give the IGCSE the same currency and credibility as its home-grown counterpart.
Teachers with experience of both invariably consider the IGCSE to be the more demanding of the two. Nor should it be forgotten that the IGCSE caters for students who move to England from international schools abroad, where the exam is the norm.
So why then is the IGCSE not a GCSE so far as the QCA and DfES are concerned? The simple answer is that, until now, neither the CIE nor Edexcel have sought approval for it from the QCA. However, neither board is likely to do so as long as they believe that the QCA will approve the qualification only if it changes its content (to become a clone of the home-grown GCSE) or its name (to differentiate it from the latter).
The first of these is out of the question, as it is the different content and style of assessment that makes the IGCSE so attractive. A change of name also has drawbacks as the link to the well-known GCSE brand is important to parents and pupils alike.
Unless the QCA gives a positive signal that it is prepared to be flexible, the IGCSE will remain the preserve of students outside the maintained sector.
A recent survey of HMC schools shows that 40 per cent are now either offering, or are thinking of offering, the IGCSE in one or more subjects.
If this trend grows, any hope of a truly inclusive 14-19 framework will disappear.
More and more independent schools would turn to the IGCSE, leaving those in the maintained sector with no choice but to make the most of whatever GCSEs emerge out of the post-Tomlinson settlement.
In policy terms, such a prospect really would be crackers.
Geoff Lucas is general secretary of the Headmasters' and Headmistresses'