More than 760,000 students took the assessment in 91 countries and economies in 2025.
A sample of schools in each country are chosen to take part from which a representative sample of pupils are chosen.
Pupils are aged between 15 years, three months, and 16 years, two months – so usually in Year 11.
It’s a two-hour online test and each student is tested in two subjects, getting different questions with variations in the combination and order of subjects.
There’s also a background questionnaire which asks for information about the students, their homes and their school and learning experiences. School leaders also complete a questionnaire.
The OECD says Pisa “measures 15-year-olds’ ability to use their reading, mathematics and science knowledge and skills to meet real-life challenges” and assesses the extent to which they have the tools “essential for full participation in social and economic life”.
It claims results provide insight for governments on their competitiveness relative to other economies.
But some experts have questioned Pisa’s value and the influence it has had in changing education policy.
After a big drop in the 2022 results the OECD said that “outcomes have largely stagnated or continued to decline in many countries”.
Overall, the steepest decline was in reading displaying a long-term “weakening of performance” which predated the pandemic, the OECD said.
Schleicher said while “limited or moderate use” of digital devices for learning in school is often linked to better results, there were substantially worse outcomes when they were used for leisure in schools.
The OECD said key ways to improve education systems included having high and consistent expectations for pupils and to ensure teachers had strong teaching skills, not just subject knowledge, as well as using AI in a targeted way which did not replace “effortful learning”.
Brychan reassured teachers and leaders that there would be no knee-jerk response to the results and plans were in place to support schools to raise standards.
“Let me be clear, these results are not where we want them to be. But in many ways, they reinforce what we knew already and highlight our need to improve standards of reading and maths, as a priority,” she said.
Brychan said the Pisa results were not the only measure of performance and standards in schools, but that did not mean they should be “ignored”, as “we take our performance in international studies incredibly seriously”.
She added: “What matters now is what happens next and delivering on our promise to better support the education system to raise standards.”
She said the Welsh government would be launching its Foundational Literacy and Numeracy Plan which represented an “important first step in the development of an equitable education system that works for all”.