Biggest achievement gaps occurring between same-ability groups within single schools.
The most striking variations in pupil achievement in Wales are most likely between classes in a single school, it was claimed this week.
A team of academics, writing exclusively for TES Cymru, claim the differences in performance between classes of similar ability can be as much as five times greater than between schools.
They blame the huge attainment gaps on the quality of teaching. In an article detailing plans for the School Effectiveness Framework for Wales, the education experts argue that narrowing the gap between the performances of different classes must be tackled to improve the life chances of all children.
This, they say, can be achieved by all schools deploying "cutting-edge knowledge on transformation and reform". But they also say the "striking and wide variations within a single school" cannot be ignored.
The radical plans for the new strategy were first revealed by this paper last month (January 18). The details were contained in a draft document, due to be officially launched by the Assembly government later this month.
This week in TES Cymru, the education experts behind it - Professor David Egan from UWIC, Professor David Hopkins from London's Institute of Education, and David Reynolds, professor of education at Plymouth University - give a full and in-depth analysis in their own words of how they expect the framework to function, defending the plans from early criticisms.
Under the proposals, Wales will be carved up into four regions for the pilot scheme, starting in May. Forty-eight schools will trial two different approaches.
Under the first, a high-achieving head, called an associate, will spend 60 days a year working on school effectiveness, paid for by the Assembly government. They will be asked to report on schools and individual performances of teachers.
Under the second arrangement, a consortium will have to produce approved attainment-raising plans for their areas.
Early criticism of the strategy has been that "coasting" schools and teachers will be placed under pressure to meet targets and raise performance. But the academics claim the strategy is not about "failing schools founded on the deployment of superheads".
Instead, they say it is designed to raise the quality of teaching in schools through a co-ordinated approach between headteachers and local authorities.
Christopher Howard, head of Lewis School Pengam and former president of the National Association of Head Teachers Cymru, generally welcomed the framework this week, but said the plans had been devised without thought to the pay and conditions of heads - or the needs of schools - deployed in the strategy.
"There needs to be immediate consultation on pay and conditions and how it will be linked with the framework," he told TES Cymru.
"I generally welcome this strategy but it does have implications."
Once released, the School Effectiveness Framework will go out for consultation.
Turn to page 29 for full and detailed analysis of the framework.