A leading educationalist has said the downward trend in exam passes in Scotland is "deeply concerning" and improvement hinges on the SNP admitting there is a problem.

Keir Bloomer, the chair of the commission on school reform, Reform Scotland, told The Herald the reduction in choice of subjects for pupils in S4 was having a knock on effect on the number of Highers and Advanced Highers being sat in subsequent years - in part leading to the drop in exam passes.

The reduction in the number of courses pupils can take is an "unintended consequence" of the Curriculum for Excellence (CfE), introduced in 2008, he claims.

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Mr Bloomer said: "It was decided that people would start their exam courses at the beginning of S4, rather than at the beginning of S3 but the Scottish Qualifications Authority (SQA) said the standards had to remain the same and that the 160 hours of study that goes into each qualification has to be done in S4, you can't do as many courses.

"When the guidance was published nobody thought of that and in my opinion, that is an unintended consequence."

He went on to say the situation was "a hallmark of bad management."

Pupils now sitting on average six subjects in a single year rather than eight over two years has lead to the loss of around a million qualifications in the last four years according to research carried out by University of Dundee academic Professor Jim Smith, said Mr Bloomer.

He said: "Changing curriculum organisation, plus changing qualifications have created a situation in which the generality of pupils are doing less qualifications.

"If you take fewer subjects, you pass fewer subjects."

The SNP have to "get out of its state of denial" in a bid to stop Scotland from lagging behind in global education standards, he said.

Exam performance is only one set of evidence that "should be worrying" the government; a triannual survey that compares student performances globally in English, maths and science, due out at the end of this year, will measure the success of Scottish students in 2018.

Scotland was found to be above average in all three areas in 2000 but went on to "decline sharply" in the two following reports, stabilising for the next two before taking a "big dive" in 2015.

Mr Bloomer, one of the architects of the CfE framework, said: "If there has been another fall, then I think the government's education policy is in very serious difficulty.

"It really won't do to keep on saying everything is rosy in the garden. It is deeply concerning for Scottish education."

Not studying vital subjects or earning high levels of qualifications is "bad for the individual and society", Mr Bloomer said, amid competitive job and economic markets.

"There is a significant hollowing out of the middle of the labour market which will accelerate as artificial intelligence kicks in, so it is becoming crucially important that an increasing proportion of the working population is capable of operating at a very high level of skill," he said.

Multi-level teaching, where secondary schools are teaching at least three levels of the same subject to pupils in the same classroom, and teacher shortages are also concerns but the 'mismanagement' of the introduction of the CfE in 2004 trumps them.

With 20,000 pages of guidance, "it was unreasonable to expect any teacher to commit this stuff to memory", said Mr Bloomer.

"The mishandling of the major educational change of our time is a big factor, " he said. "After the production of the 2004 document, the necessary preparation of the ground never took place. Blame lies in the quality of advice that governments, plural, have received over the years."

To turn things around more transparency is required, said Mr Bloomer: "there's got to be a recognition there's a difficulty - the government has to stop pretending that problems don't exist.

"There's nothing wrong with scrutinising good evidence of what is happening and accepting there are places where policy needs to be adjusted. I think that's an honest and reasonable way of approaching things."

Simplified CfE guidance is set to be published by Education Scotland in the coming weeks which will "refresh the narrative" and be a"major step forward" for the system, he said.

The Educational Institute for Scotland's general secretary Larry Flanagan is concerned that critics are "naval gazing" at the further drop in pass rates this year.

He said: "It seems Scottish Education can’t win – a Higher pass rate at Grade A-C of 74.8% is greeted not with applause for a very high performance from our schools and pupils, but a ritual wringing of hands about the “fall” of 1.9% from the previous year.

"If there had been an increase of a similar percentage, undoubtedly critics would be talking about exams becoming easier and grade inflation having taken hold, a phenomenon that Scottish Education has guarded against."

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He called for an interrogation of the impact of the extended exams, as well as detailed analysis of each individual subject to identify the cause of the downward turn.

He said: "A sense of perspective is required. Firstly, the drop is not uniform – some subjects are demonstrating an increase whilst some subjects where teachers had raised concerns about changes to course content are demonstrating a bigger drop – Computing Science, for example, has seen a drop of 4.9%. What we need is a bit of stability."

He went on to say: "On a positive note there has been a significant drop in the number of 'No Awards' suggesting an improved alignment of presentations with passes."