What does today's Programme for International Student Assessment (Pisa) report tell us? As far as England is concerned, not much has changed since 2006. The average scores in science, maths and reading have remained stable.
Given that there has been a tidal wave of education reform in the intervening decade, people are entitled to ask why this has not propelled us up the rankings.
We have seen ever more stringent accountability measures, the wholesale restructuring of the school system, and significant changes to the curriculum and qualifications.
So what does Pisa 2015 tell us about the success or otherwise of these changes? Has it been worth all the blood, sweat and tears? And are we heading in the right direction?
It has, at least, not had a negative impact. That is not an entirely flippant observation. The huge turbulence caused by these waves of education reforms could easily have had a negative impact, as schools struggled to cope with constant change and disruption.
It is to schools' great credit – and thanks to an enormous amount of hard graft – that this has not happened.
However, the point of the reforms is obviously to produce a positive result and that is where it becomes harder to reach a conclusion.
'Pisa isn't the whole spectrum of education'
Pisa – while an important international study and one which certainly generates a lot of headlines – is only one measure of success.
The curriculum is much broader than maths, science and reading. It encompasses humanities, languages, arts and technical subjects, physical education, life skills and enrichment activities.
Its purpose is to produce rounded, resilient young people who are on clear pathways through to further and higher education, and their future careers.
Pisa cannot possibly measure the entire spectrum of education. We need to look for other broader measures to see if standards have risen during that time.
One such measure is the number of schools which are judged "good" and "outstanding" by Ofsted. And it is notable that this now stands at a record figure of 89 per cent.
Whether you attribute this success to the reforms which have taken place over the past 10 years – which of them you think have worked and which have not – depends on your perspective.
Many have, of course, been enormously controversial. And the most significant factor may well have been the sheer hard work, energy and professionalism of hundreds of thousands of teachers and school leaders over the course of that period.
Whatever your view though, most of us would probably agree that there has been an upward trajectory in educational performance, not only over the past 10 years but going back over the course of the past 30 years.
There have undoubtedly been bumps along the way, but the direction of travel has been the right one.
And this brings us back to Pisa 2015 because one of the most interesting findings in relation to England is about the impact of teacher shortages.
Almost half of secondary school pupils in England are taught in schools where the headteacher believes that staff shortages are hindering learning. This is 15 percentage points above the OECD average.
We know from the McKinsey report How the World’s Best Performing Systems Come Out on Top that: “The quality of an education system cannot exceed the quality of its teachers.”
It follows that unless we put a much greater focus on teachers, we simply cannot expect to take the next big leap forward in our education system. We need to recruit more teachers. We need to retain them. We need to pay them properly. And we need to support them with world-class professional learning programmes.
This is not a job for government alone. It is also a job for the profession.
But it is the government that has to initiate the process so that we can work together productively to find solutions to the current teacher supply crisis and develop the next generation of great teachers and great leaders.
It requires an overarching strategy, not piecemeal initiatives, and the time to start is now.
Leora Cruddas is director of policy at the Association of School and College Leaders. She tweets as @LeoraCruddas