Headteachers and teachers in Wales are being urged to provide false information as part of the colour-coded performance ratings giving an inaccurate picture of a school achievement, the Guardian can exclusively reveal.

Assembly Member Adam Price has now raised concerns over the National Schools Categorisation System – the process by which schools receive a colour-code to determine whether or not they need additional support.

The Carmarthen East and Dinefwr AM met with one local head teacher over the summer who raised with Mr Price a 'loophole' in the categorisation system which enables pupils to be noted in an alternative curriculum year to their actual age group – a step, the headteacher says, which gives an inaccurate picture of a school's overall attainment.

Mr Price went on to make initial representations to the Welsh Government's Cabinet Secretary for Education who has since acknowledged that such a void exists. The Cabinet Secretary did however express that there was no evidence to suggest systemic manipulation of the data.

Mr Price has since been made aware of teachers who state they have been expressly asked to take such steps.

The Plaid Cymru AM has now called on the Cabinet Secretary to take all necessary steps to better support schools in their categorisation process.

Adam Price AM said: "The principle behind a school's categorisation is to identify where additional support can be provided. However 'support' can sometimes turn into a disproportionate increase of monitoring, added pressures and risk of intervention.

"With a school's categorisation often then being incorrectly used as a form of league table, a significant burden is placed on teachers and head teachers to improve their figures.

"We want an education environment in which teachers are not pressured and head teachers are comfortable in being transparent in their data. This is the case in the vast majority of schools. But I have been made aware over the summer of the potential to influence categorisation figures. This doesn't help schools, pupils or teachers in the long term.

"Better supporting schools has got to be the rationale for categorisation. This requires everybody to work in good faith, and the Welsh Government assisting schools to correctly undertake the process."