International comparisons with our education system often generate more heat than light in the debate about standards. The comparisons are not useless, but pieces of data tend to be plucked and used out of context.

The same data, considered more thoughtfully, can reveal a lot. Last week was a case in point.

Why is it, Tony Blair was asked at Prime Minister’s question time, that the latest Organisation for Economic Co-operation and Development data seem to show standards falling, with the number of students completing secondary school lagging behind that of other countries?

A well-briefed Prime Minister pointed out that the figures do not relate to qualifications gained under New Labour. They show only that the current crop of 25 to 34-year-old Britons are less well-qualified at secondary levels, compared to their international counterparts, than are the members of an older, 55 to 64-year-old generation of Britons.

A more up-to-date picture, Mr Blair argued, comes from our 15-year-olds’

strong performance when tested under the OECD’s programme for international student assessment (PISA).

However, this argument misleads. It crucially mixes up two very different things. PISA measures levels of mastery and application of skills like reading and mathematical literacy. The other OECD data simply measures how many students gain qualifications and remain in full and part-time education.

The numbers show that in recent decades some countries have overtaken the UK in the proportion of students leaving secondary school with decent qualifications.

The UK has not “declined” in this area. It has simply failed to progress as quickly as other European countries in extending post-16 participation from a minority to almost all of the youth population.

New Labour has yet to turn this relative failure around: at present more than a quarter of our 17-year-olds are still outside education. This is higher than in all but four of the 30 OECD countries, where in some cases more than 90 per cent are staying on.

So how does this relate to PISA’s comparison of skills? The survey shows that reading and mathematical literacy are stronger among British 15-year-olds than their peers in most other countries. This is true not only on average, but also among the relatively less able.

The striking conclusion is not that British students have low standards of performance, but that those who perform relatively well are dropping out of education.

Compare and contrast, for example, students who are near the bottom of their nation’s educational pile, on the 25th percentile.

In terms of reading scores for 15-year-olds, this relatively low-achieving student in the UK gets a fairly respectable PISA score of 458 - surpassed by equivalent students in only six of the 27 OECD countries in the survey.

(The average PISA score, internationally, is 500; two-thirds of students score between 400 and 600.) But in terms of how long they stay in education, the British student fares less well. By the age of 17, this student has left full-time education and training, probably with some low-grade GCSE passes and scraping a C in one subject.

In Germany, the equivalent student has a much lower PISA score, 417. But the 17-year-old German is on an apprenticeship scheme with part-time education. In Sweden, the student has a similar PISA score to the British student, but remains in education until age 19 to complete full-time upper-secondary school on a vocational track.

The comparison shows the precise opposite of the claim by the “standards-are-falling” brigade that the riff-raff are being given access to further and higher education. On the contrary, in this country students who demonstrate potential and ability are not staying on at the same rate as their counterparts in other countries.

Why not? The PISA study shows some evidence of poor engagement in learning among British students - their reading habits and attitudes are relatively weak overall - but not enough to explain such a high drop-out.

A stronger clue is the higher than average association in Britain between student performance and social background. This suggests that a hard-to-measure factor - poverty of expectation - continues to hold powerful sway.

It might be an old chestnut, but no less important for that: the class system is still holding back students who have not yet realised that, by international standards, they are really doing rather well.

Perhaps if the press were not quite so fond of nuggets of bad news, it could help them to wake up to this fact.

Primary forum 32 Donald Hirsch is an international consultant on education policy