Ruth Davidson has threatened to withdraw her party's support for the government's Curriculum for Excellence reforms after Scotland dropped down international education rankings for core school subjects.

The Scottish Conservative leader said the SNP's flagship school reforms were "on probation" in the wake of the results of the 2015 Programme for International Student Assessment (Pisa).

The survey of 15-year-olds found Scotland's performance in reading, maths and science had declined relative to other Organisation for Economic Co-operation and Development (OECD) countries and UK administrations.

READ MORE: Government agency deleted student bursary reference in report after SNP Government intervened

Scotland was ranked above the OECD average for reading and science in the 2012 study, and at average in maths, but is now similar to the average for all three subjects.

At First Minister's Questions at Holyrood, Nicola Sturgeon accepted the results were "not good enough" but insisted the reform of the curriculum had been the right approach.

Referring to the government's ongoing review of school governance, Ms Davidson said: "Given the evidence of this week, I think it has to go deeper.

"The single biggest education reform under this SNP government has been Curriculum for Excellence and nobody here can simply brush aside the fact that since it has come in standards have fallen.

"So, I'm telling the First Minister today that our ongoing support for Curriculum for Excellence cannot be taken for granted.

"I believe that this entire project should be put on probation and there's a simple question that I ask, and I ask it in all sincerity: If standards are going down because of it, why are we sticking by it?"

In response, Ms Sturgeon read a quote: "The principles behind the Curriculum for Excellence are absolutely right, there is unanimous agreement within the parliament that it is the right way, as there is within the teaching professions, and we have to accept that this is something that can work."

She continued: "These are not my words, these are the words of the Tory education spokesperson Liz Smith on December 7.

"I believe Curriculum for Excellence is the right way forward and indeed Ruth Davidson has rightly and understandably quoted today the Pisa survey, a survey carried out by the OECD.

"We asked the OECD, the very same organisation, to review Curriculum for Excellence.

"It has said Curriculum for Excellence is the right thing to do but it has given us advice ... about how we improve it and how we improve standards in our schools."

However, reading from the OECD's report, Ms Davidson quoted: "We emphasise that this summary is not an evaluation of Curriculum for Excellence and indeed the evidence is not available for such an evaluation".

She said: "We have had all the while from this government bland platitudes and all the while those platitudes have been coming, standards have gone down.

"We have gone backwards in reading, backwards in science, backwards in maths and this week the SNP, the First Minister's colleagues, told us that despite all the challenges we face right now, what would be good for Scotland is a constitutional crisis.

READ MORE: Government agency deleted student bursary reference in report after SNP Government intervened

"It's not another constitutional crisis we need, it's a government that faces up to its failures and that tackles them head on.

"So, I ask the First Minister: what's more important, picking yet another fight over the constitution or picking a fight to improve our schools?"

The Tory leader also asked the First Minister who she would blame for the "worst set of results ever recorded" in order to "pre-empt the usual excuses".

Accusing the SNP of being like a "stuck record" on the issue, she added: "We've had 10 years worth of promises by education secretary after SNP education secretary.

"How does the First Minister mark the results of their efforts, pass of fail?"

Ms Sturgeon said: "I take responsibility on behalf of the Scottish Government for the performance of Scottish education and if anybody thinks I'm going to stand here today and give any excuses, they are wrong.

"There's lots of other evidence I could cite about Scottish education but I'm not going to do that today because the results of the Pisa survey earlier this week are not where I want us to be, they are not good enough and I am determined that we take the action that will lead to improvement."

She highlighted improvements including a record number of exam passes in Scottish education and a higher percentage of young people going into positive destinations.

The First Minister listed actions recommended by the OECD that the government was taking to improve performance, including the Attainment Scotland Fund to narrow the gap between rich and poor children, the introduction of standardised assessments, measures to simplify the curriculum and the ongoing review of school governance.

READ MORE: Government agency deleted student bursary reference in report after SNP Government intervened

She said: "These are hard, concrete, tangible actions and I know the opposition will want to criticise for the Pisa survey, and I can have no complaint about that, but what I think is most important now is that we all get behind these reforms because these are reforms that will lead to the improvements we want to see in Scottish education."

Ms Sturgeon added: "It may have escaped Ruth Davidson's notice but the entire UK faces a constitutional crisis right now.

"It's being played out in the Supreme Court this very day, as it has been all week, and it's a constitutional crisis created by the European obsession, the Brexit obsession, of the Conservatives."

Scottish Liberal Democrat leader Willie Rennie listed more than 20 countries placed higher than Scotland in the education rankings, including South Korea, Denmark, Ireland and Poland.

Demanding an explanation of "what has gone wrong," he said: "After 10 years of SNP rule, we are not even as good as England any more."

Ms Sturgeon told him she was "not going to rise to the bait on the politics of this issue".

Mr Rennie pointed to statements made by members of the Scottish Government on education and said "complacency has been oozing out of ministers for a whole decade" while they "presided over the worst results ever".

He went on: "They prefer the reintroduction of Thatcherite school league tables, the centralisation of education and 20,000 pages of guidance.

"So, will the First Minister over-rule all of these ministers and invest the £500 million for education in the Budget next week?"

Ms Sturgeon said OECD findings show that investment is important but is not, in itself, enough.

She told the chamber: "It makes the point that the difference in Pisa scores between countries is less to do with variation in investment and more to do with differences in policy and practice.

"So that is why, yes, we will invest, but we will also make sure we are carrying forward the reforms to our school system that are required to be made."

Speaking after First Minister's Questions, Education Secretary John Swinney said: "Ruth Davidson showed her true colours once again today.

"The Pisa results made for uncomfortable reading - but in difficult times leaders do not abandon their principles, retreat and run for cover. They hold fast to what is right.

"Now, just as we have seen with her sudden conversion to a hard-right Brexit, Ruth has revealed she cannot be trusted to stand by what she says she believes.

"In contrast, her own Education spokesperson, Liz Smith, is right when she said, just yesterday, that CfE is the way forward for Scotland's schools.

"Sadly, Ruth seems less interested in pupils' education and more interested in pursuing the next cheap headline.

"Her attack on CfE reveals a lack of substance and of political resilience. It does her no credit at all.

"The Scottish Government will remain firm in our beliefs and listen, not to the turn-and-run Tories, but to our international advisers who have told us to be urgent in what we do, but patient about results. We will stay the course."