First minister Nicola Sturgeon has said she is "not denying" the challenge of improving education in Scotland following the publication of an international survey on performance standards.
The Programme for International Student Assessment (Pisa) reports published yesterday show a rise in reading performance but no improvement in science and maths.
Pisa figures are produced every three years and record the performance of 600,000 15-year-olds from around the world.
Pisa results 2018: What happened in Scotland?
Disadvantage: No sign of attainment gap cash 'making an impact'
Pisa: Are the Scottish and English education systems less different than we thought?
Global rankings: 2018 Pisa results reveal a mixed picture
In the latest results, Scotland achieved a mean score of 504 for reading, up from 493 in 2015; 489 for maths, similar to the 491 score in 2015; and 490 for science, down from 497 in 2015.
Speaking today during a general election campaign visit to the Happy Days Nursery in Dalkeith, Midlothian, Ms Sturgeon said there was a "wealth of evidence" that education standards are improving.
Pisa 'shows focus on literacy'
The first minister said: "What we saw yesterday was a very sharp and very significant improvement in reading performance, and that comes from a determination after the last Pisa results to make a focus of our attainment challenge on literacy initiatives like the First Minister's Reading Challenge.
"Performance in maths and science is stable – that was the word that the OECD [Organisation for Economic Cooperation and Development] used – and the initiatives that we have underway now are all about making sure that we push up performance there as well."
Asked about the performance in science and maths, Ms Sturgeon said: "I'm not denying the challenge here but it is a point of fact that performance is stable compared to the last Pisa survey."
The first minister said: "Performance is stable and what we are seeking to do now is see it improve in the same way that reading performance has improved over the last couple of years
"Now, there is a wealth of other evidence, if you look at the performance in Higher exam passes in Scotland, both in terms of the numbers of young people leaving with Highers, the closing of the attainment gap in Higher results...then we see a wealth of evidence now that standards are improving and the attainment gap is closing.
"It's not job done and I am not standing here saying it is but there's a real focus on the part of the Scottish government to make sure that that improvement continues."
She added: "Of course, we see record numbers of young people, from all backgrounds, now going into positive destinations, we see more young people from our most disadvantaged communities now going into university – that was, when I became first minister, a particular challenge that we've really addressed.
"So I'll be candid, there is work to be done here but in many of these respects the direction of travel is the right one and our responsibility is to keep that focus strong."
Yesterday, Scottish Conservative education spokeswoman Liz Smith called the statistics "damning" for the SNP's record on education.
She said: "This summer saw the fourth consecutive year of decline in Higher pass rates and now the SNP is presiding over the worst-ever Pisa results in both maths and science.
"In reading, where it should be acknowledged that there is encouraging improvement since 2015, the score is still lower than the 2012 result and considerably lower than the score in 2000."