The banding system used to rate secondary schools in Wales has been described as "flawed", "crude" and "confusing" after the annual ratings were published this week.
Each of the nation's 219 comprehensives has been placed in one of five bands based on measures including attendance and GCSE results, with band 1 being the best and band 5 the worst.
It is only the second time that the rankings have been published, after their launch last year. But the results have generated serious concerns about the methodology used and the accuracy of the data.
It emerged that 10 schools have fallen from the top band to either band 3 or 4 this year. Among them is Treorchy Comprehensive in Rhondda Cynon Taf, which - despite being rated excellent by Estyn in an inspection earlier this year - dropped from band 1 to band 3.
Likewise, Ysgol Tryfan in Bangor dropped from band 1 to band 4 because fewer pupils gained the level 2 threshold of five GCSEs at grade A*-C including English or Welsh and maths. The headteacher said neither year was a fair reflection of the school's overall performance.
This year's data revealed that 72 schools have moved up at least one band, 71 schools have moved down at least one band and 75 have stayed the same; one school was not banded last year. The system only allows for a fixed percentage of schools to be in each band. The Welsh government is also pressing ahead with plans to introduce a similar system to rate primary schools (see panel).
Teaching unions questioned whether the system was playing its part in raising school standards, given that more than 40 per cent of schools in the lowest two bands have stayed in the same place or dropped a band despite each receiving an extra #163;10,000 to help them improve.
Gareth Jones, secretary of heads' union ASCL Cymru, said that school leaders would find the data "depressing and demoralising". "We have no problem with the government publishing the data, but the flaw is condensing that down to a single band," he said. "That's what the public will see and there will be confusion. In our view an Estyn inspection is a far more rounded judgement of a school's performance."
NUT Cymru called the system a "total failure" and urged the government to "recognise its mistakes". "It is hard to see how a system that has so little support among the profession can continue in its current format," David Evans, the union's secretary, said. "It is causing real difficulties for schools and, unless that is addressed, risks creating a long-term crisis for Welsh education.
"We only need to consider the fact that the school ranked at the top of band 1 last year is now finding itself in band 4 to see that this is a seriously flawed system."
Welsh teaching union UCAC also called on the government to reconsider the system, saying it has turned "sensible, valuable data" into "arbitrary and misleading figures".
Conservative shadow education minister Angela Burns said the government should take the "flawed" system back to the drawing board.
But the government is sticking to its plans, having made banding a central plank of its education manifesto at the last Assembly elections.
Education minister Leighton Andrews said the data are "robust" and provide a clear picture of school performance and the information needed to challenge schools that are not delivering.
"Banding is at the heart of our school improvement agenda in Wales and we're committed to releasing this performance data year on year," he added.
Education minister Leighton Andrews has decided to press ahead with controversial plans to band primary schools in Wales, despite widespread opposition.
The government was forced to shelve its plans to introduce the system last summer amid concerns that data from key stage 2 teacher assessments were not sufficiently robust to give an accurate picture of performance.
But the minister recently confirmed that primary banding will recommence in September 2014.
He said the government is currently drawing up a profile of each primary school and will discuss their individual support needs with regional consortia in the coming months.