The publication last term of the examination results and absence rates for all secondary schools is a watershed moment for education in Wales. In England - rightly or wrongly - the public are bombarded with school performance data for all primary and secondary schools, now with new value-added measures to boot.

By contrast, in Wales national data on individual school performance has not been published since 2001, with the great approval of schools and most education professionals. Now we can see the statistics, what are they telling us?

First, they show the enormous difference that the individual school makes to its children. Schools with similar proportions of pupils entitled to free school meals - a surrogate for their social backgrounds - are achieving dramatically different results.

For example, if we look at schools with 30 to 35 per cent free school meals (FSM), the pass rate for five or more GCSEs at grades A*-C range from 20 to 50 per cent - a gap of a staggering 30 percentage points. Sandfields, in Port Talbot, and Lliswerry, in Newport, are the stars here who shine out from the rest.

Also, in adjoining local authorities in south Wales, there are schools with the same FSM rates but with the same 30 percentage points difference in their GCSE success rates.

There are also schools with similar FSM rates within the same local authority, and therefore receiving the same central services in areas like school improvement, that differ by up to 20 percentage points in their GCSE figures.

And there are local authorities that are rapid improvers, Neath Port Talbot being the star, and others that are not. The figures suggest Wales exhibits a world of difference - both between schools and between local authorities.

This may not be a negative thing, if the difference is being caused by schools at the leading edge constantly moving ahead of the others. But this may not be the case in Wales. Something positive may also come out of it, if we are learning from the schools that are excelling, but this also may not be happening in Wales.

All that can be said is that the quality of their children’s education for the parents of Wales is a lottery, depending upon the school they choose, and the local authority they live in. Clearly, this is unacceptable.

Second, the statistics appear to show that underachievement is happening not only in Wales’s disadvantaged areas - the perennial explanation - but in its more advantaged areas too.

Those schools in Wales in the 2 to 5 per cent FSM range are getting up to 70 and 80 per cent pass rates for five or more good GCSEs. But schools like these in England are knocking on the door of 90 per cent.

Indeed, England now has more than 100 schools with every pupil getting five or more A*-C grades, a group which includes many schools with the same social conditions as our lower-scoring Welsh ones.

The other phenomenon with some of the Welsh high performers is flatlining - the tendency for schools already doing well to stay at their high level of performance rather than improve on it.

We would be expecting these schools to be improving at the rate of all Welsh schools in their success rate, but many are not. These coasting, idling - or what some have nicely called promenading - schools deserve attention.

Third, the figures show that the Welsh-medium sector - the jewel in the crown of education in Wales historically - may now be having problems.

Its performance in 20045 was actually worse than in 19992000, probably because of the difficulties the schools faced adjusting to new intakes from a broader variety of social backgrounds from the mid-1990s.

A number of these schools in more disadvantaged areas are clearly struggling and they need some help.

These data raise more questions than they answer. Using FSM rate as the measure of the quality of school intakes is not a perfect method.

And it has only been possible to look through, or eyeball, all the schools results, rather than statistically analyse them.

But there are enough differences between schools and local authorities on show, and enough concerns about the advantaged and Welsh-medium sectors, to warrant a serious investigation into the state of Welsh secondary education.

Are the present mechanisms for spreading best practice between schools adequate? Are local authorities fulfilling their school improvement roles? Are the Assembly government and Estyn doing enough to shed light into dark places?

And if these simple data can provoke so many important, unanswered questions, what lurks in the dark of the other Welsh data which is yet to see the light of day, such as that on the results for our primary schools? We need to know.

David Reynolds is professor of education at Plymouth university and emeritus professor at Exeter university. He lives in south Wales