Ruth Davidson has accused Nicola Sturgeon of "rigging" official education statistics to "con" parents that pupils' literacy and numeracy have markedly improved.
The Scottish Tory leader said that under a new system for measuring performance pupils can be deemed to have met the required standard even if they fail their maths and English exams.
She accused the SNP government of "lowering the bar" by scrapping the Scottish Survey of Numeracy and Literacy (SSLN) in order to "massage the statistics to make themselves look good."
But Ms Sturgeon accused Ms Davidson of being "very confused" and claimed the survey had been replaced by a system that was more "comprehensive".
The row broke out at First Minister's Questions after statistics published this week showed show 80.8 per cent per cent of school leavers in 2016/17 attained literacy at National Five standard - up from 70.1 per cent in 2013/14.
For numeracy the standard was met for 68.8 per cent of pupils, an increase from 59.5 per cent over the same period.
The final SSLN, published in May last year, found only 49 per cent of S2 pupils could write well, down from 64 per cent in 2012. For reading, the proportion had dropped by two points to 80 per cent over the same period.
Scotland also recorded its worst ever performance in an international survey published in December 2016, tumbling down a league table for maths, reading and science and falling behind some other parts of the UK.
Ms Davidson said: "It used to be the case that we could measure literacy and numeracy standards fairly with accurate surveys but when it turned out the rates were going down the SNP cancelled them, and we now have a new system in place.
"Now the First Minister keeps saying that she wants to boost standards, so how does cancelling surveys, how does rigging the stats, how does lowering the bar for literacy and numeracy help achieve those higher standards?"
She accused the Scottish Government of "utter complacency" towards education reform, saying it had "cooked up a new measure of attainment in literacy and numeracy to try and con parents into believing that things are getting better".
The Scottish Tory leader also highlighted an investigation that found some state schools have not been inspected for 16 years and one in five have not been visited for at least a decade.
I wish someone could persuade the FM to stop implying well-constructed sample surveys (as SSLN was) are inevitably a second best to population wide sweeps, however inconsistently applied. https://t.co/aVXwZjMKWH— Lucy HunterBlackburn (@LucyHunterB) June 21, 2018
Ms Sturgeon said: "Ruth Davidson is just wrong in much of what she said there. Nobody is cancelling tests.
"What we have done is replace a sample survey with comprehensive data on the performance of pupils right across the country. We've taken a survey that looks at a handful of pupils and replaced that with data on every pupil across Scotland."
The First Minister said inspections are due to increase in the 2018/19 academic year, with 250 schools being visited.
However, Lucy Hunter Blackburn, a former Scottish Government mandarin in charge of higher education, undermined Ms Sturgeon's defence for scrapping the SSLN.
She tweeted that Ms Sturgeon should "stop implying well-constructed sample surveys (as SSLN was) are inevitably a second best to population sweeps, however inconsistently applied."