The latest Pisa international education report has found a long-term decline in Scotland's performance in reading, maths and science.
The fall is larger than in England, the UK as a whole, and the OECD average.
Pamela Nash, chief executive of Scotland in Union, said:
“Scotland’s education system was our jewel in the crown, internationally renowned.
“Nicola Sturgeon told us to judge her government on its record in education.
“But after 16 years of the SNP in charge, our children’s schooling has been unforgivably diminished.
"This is the devastating consequence of the SNP's focus on the constitution at the expense of our children's futures.
"These findings must act as an urgent wake-up call for the government to focus on what really matters.
“It’s time for the people’s priorities, not the SNP’s"