Complacency and a lack of ambition from pupils has led to Wales being the worst performing in the UK in international education tests, it is claimed.
Organisation for Economic Co-operation and Development (OECD) head of education Andreas Schleicher said pupils must try harder.
Mr Schleicher's comments come as First Minister Carwyn Jones gives a speech in Cardiff on the evaluations.
The education chief told BBC Radio Wales: "I think, in part, it's because other education systems have improved their performance and in relative terms Wales has declined.
"It's also been an actual slippage and there's sort of less aspiration and less ambition in the system.
"You can expect a lower performance from disadvantaged schools but the fact that even schools in well-off areas of Wales don't live up to other schools in similar conditions shows that there is a bit of complacency in the system."
Wales has been the worst performing UK nation in the three-yearly tests prompting a drive to improve standards.
'Demand for better skills'
Mr Schleicher criticised Wales for a lack of long-term vision for education.
He said: "Your education system today is your economy tomorrow.
"There's a very, very close linkage between the skills that people develop in school and what they're able to do later in life. Our economies are evolving very rapidly. The demand for better skills, for the right skills.
"The knowledge economy no longer pays you for what you know... it pays you for what you can do with what you know."
He also criticised the mindset of Welsh students.
He said: "We ask them a simple question - what do you believe makes you successful in mathematics?
"You find many students in Wales who tell you, 'well, it's all about talent, if I'm not born a genius in mathematics I'd better study something else'.
"If you ask the same question to a Chinese person or a Finn, students will tell you, 'if I try hard and trust my teachers to help me, I'm going to be successful'."
Mr Schleicher said the culture of education must change.
He said: "The fact that many countries on Pisa have seen significant improvements - if you look at Poland in Europe, or Germany in Europe or some countries in Asia. The fact that they are improving shows that you can change that culture."
Wales was ranked bottom of the four UK nations and fell further behind other countries, with east Asian cities and nations coming out on top.
Reforms including statutory reading and numeracy tests had already been introduced in Wales following previous disappointing Pisa results.
'Pride in education'
Speaking before the conference on Wednesday, Mr Jones said he wanted to put "pride back into our education system".
Education Minister Huw Lewis added: "Reform in education does take a long time, we've come a long way in terms of school banding, our literacy and numeracy framework, making sure those key elements... are addressed head on."