England’s secondary schools have just been confronted with the second unexpected publication this year of “hidden” data about their exam results.

Heads, still reeling from January’s retrospective application of the English Baccalaureate (EBac) measure to their 2010 GCSE results, have been hit again. And this time it’s an avalanche.

A total of 14 million lines of data about individual schools were published by the Government two weeks ago.

They allow anyone with access to the internet to see the number of children entered for each GCSE subject and every grade they achieved in every secondary. The new data also reveals, for the first time, the proportion of pupils in each school achieving five A*-C GCSEs, including English and maths measure without “equivalents”.

Ministers say they are bringing greater transparency to the schools system. But many heads will feel sore because they believe the goalposts have been moved unfairly.

They entered pupils for exams last year expecting to be judged as they had been since 2006 - on the proportion of pupils in every school achieving the five A*-C GCSEs or equivalent.

That headline measure had become the be-all and end-all for school league tables. Do well and you could be lauded as an improving school, receive a glowing Ofsted report and generally be left to your own devices.

But do badly and you would be named and shamed in the media, condemned as failing by ministers and, ultimately, closed or taken over.

The rules were simple and, for good or ill, both schools and government knew where they stood. Getting a respectable score on the headline measure was everything, and no one much examined how you got there.

Even when the measure was changed - the Labour government only decided to add the English and maths GCSE requirement in 2005 - it was piloted first and schools were given plenty of notice.

But this year, the coalition Government tore up the rulebook. The EBac - requiring grade C GCSEs or IGCSEs in English, maths, two sciences, a language and history or geography - was introduced out of the blue and immediately applied to 2010 results.

Suddenly, there were two headline measures. But schools had had no time to prepare and many have been frantically trying to catch up ever since.

Special EBac training days have been organised, some schools have quickly switched pupils onto EBac-friendly courses while others have pressurised them into attending extra after-school classes.

Then, in the last fortnight, there has been another league table earthquake. With very little warning, ministers ripped away the veil of “equivalent” qualifications, leaving some schools very exposed on the traditional benchmark of five GCSEs at A*-C, including English and maths.

By now, heads could be forgiven for being in a bit of a spin. A third new measure has been unleashed on them while they were still trying to come to terms with the second.

This flurry of measures is not the product of haphazard policy-making. It is a deliberate strategy. “An over-reliance and concentration on one single measure can sometimes mean that we are not getting the full picture about how schools and the education system is performing,” education secretary Michael Gove has said.

But the new develpment goes beyond simply providing more information. Ministers also want to change schools’ behaviour.

Sources close to Mr Gove argue that having multiple measures is a good thing because it reduces “gaming” of the system - schools adopting qualifications simply to climb the league tables.

The Government’s case that “gaming” is taking place was put in the Wolf report on vocational education, published last month.

It found an “explosion” in schools’ use of vocationally related qualifications with GCSE “equivalences”. In 200304, just 1,882 such qualifications were achieved. By 200910, the figure had rocketed to 462,182.

Writing in the report’s foreword, Mr Gove concluded: “Far too many 14-to-16 year olds are doing courses with little or no value because performance tables incentivise schools to offer these inadequate qualifications.”

Heads’ leaders deny that schools have been using “soft options” to boost league table positions. Brian Lightman, general secretary of the Association of School and College Leaders (ASCL), said that the drop from 53.4 to 49.2 per cent on the five A*-C GCSE benchmark after equivalent qualifications were stripped out could “hardly be called shocking”.

But it is a national figure, an average, that glosses over much more extreme examples where the removal of vocational courses has led to schools’ benchmark scores plummeting.

It is not unusual for a head to admit privately to “playing the game“when it comes to selecting qualifications.

Even the ASCL acknowledges that a culture has grown up where many heads take decisions on the basis of how they believe their school will be judged by officialdom, rather than what is in the best interests of their pupils.

The panicked reaction to the EBac is a case in point. If schools really believed the qualifications already on offer were the ones that best suited their pupils, why are so many changing them so quickly?

It is not as if there are any plans for the EBac to be used by Ofsted or adopted for official “floor” targets. Mr Gove has said repeatedly that the five A*-C GCSEs will remain the key benchmark and the EBac will remain an “aspirational measure”.

But try telling that to the head of a school with a deprived intake who has pulled out all the stops to scrape past the latest floor target. They will be listening out for any change in policy, acutely aware that a slip could easily lead to ignominy, closure and redundancy.

Successive government improvement drives, naming and shamings and fresh starts have created a highly reflexive and sensitive schools system, programmed through experience to respond instantly to the slightest signal from the top.

And those signals are multiplying. Government sources have already told The TES they would be interested to see how schools rate against an “advanced Bac” - EBac plus triple science and English literature - though there no immediate plans to publish such a measure.

More data releases will soon allow anyone with an interest in education to construct their own measures in a process that ministers believe will help drive improvement.

The Government is also considering an “accelerated Bac” for schools that skip GCSEs in favour of A-S levels. And there are suggestions of new measures at 16-19 - perhaps a league table based on A-levels in maths, further maths and physics.

“Who knows what we might choose to stress next time round,” a source said.

There are those in Government who believe that many of the heads who complained about the retrospective introduction of the EBac were simply angry that they had not been given enough time to “con” the latest version of the league tables.

And this, from the Government’s point of view, is the advantage of a wider variety of measures. Schools would find it impossible to “con” them all at the same time. So league tables can be rid of their perverse incentives and complete transparency be introduced to exam results.

But there are significant potential flaws. The first is that not all of these measures will have equal weight. The traditional GCSE benchmark remains the “anchor” high-stakes measure on which Ofsted judgments and crucial floor targets rest.

Once the fuss surrounding the EBac dies down, some schools could revert to concentrating on the one measure that really matters to their futures - and all the perverse incentives will remain.

Much will depend on how ministers respond to the Wolf report, which did not recommend that “equivalent” vocational qualifications should be stripped out of the GCSE benchmark altogether.

Instead, ministers are currently considering which qualifications should still count towards the measure and what value they should have. If they opt just to tweak it, it is possible that there would be very little change in the qualifications that schools use.

The knowledge that parents, media and the public will now be able to see how schools perform on the alternative measures could act as another disincentive to “gaming”.

But for that to happen, the measures will have to capture the public imagination and be widely understood. The media response to the latest data release was probably more muted than ministers had hoped, and local newspapers - crucial to school admissions - virtually ignored the EBac.

But that has not stopped the composition of the measures from being highly contentious. Mr Gove has always said that there will be room to offer other subjects and courses alongside the EBac. But some fear schools will concentrate all their efforts on the Government’s new priorities and end up with a much narrower curriculum unsuited to many pupils.

The “Build a Better Bac” campaign - currently devising an alternative to the EBac - is a sign that not all schools unquestioningly dance to the Government’s latest tune.

Ministers hope that the newly released data will provoke a similarly creative reaction from teachers, parents, unions or newspapers, who will be inspired to create their own school measures.

But that will only be worthwhile if the Government pays any attention to them. High-profile academy chains praised by Mr Gove for “driving up standards” do not do well when judged against his favoured measure of “academic vigour”, the EBac.

Will Mr Gove be happy to sing their praises if their performance does not improve on this metric?

Then there is the question raised by Mr Lightman of whether there is any desire among parents to spend time conducting their own audits of exam results.

The Government says its new league tables will allow parents to check whether schools offer what they want. They would, for example, be interested to find out whether a school had a particular strength in geography, a Department for Education official asserted last month.

But will they really? At GCSE level, what most parents want to know is whether schools offer a good all-round education.

In that sense, the data will help. Parents may not be searching for GCSE geography centres of excellence, but they will expect schools to offer the exam.

Thanks to the Government’s latest data release, we now know that 137 of them don’t.

The new league tables may not achieve all that ministers would wish, but revelations like that will demonstrate a greater transparency that many will welcome.