Teachers' leaders are "unimpressed" by the Government's proposal for school report cards as an alternative to league tables.
The warning comes in the NUT teachers' union response to the official consultation that is on the cards, which would give schools an overall grade or traffic light rating based on a range of measures much wider that exam results alone.
The union says: "There is a real danger that the introduction of the report card will simply reduce schools' work to a checklist of indicators, which does not capture or simply ignores the real work, often innovative, that schools have been doing for some time now to improve the life chances of their pupils."
Inspired by what are known as balanced scorecards in the US, the school report cards could include parents' and pupils' views, Ofsted ratings and measures of pupil health and safety, enjoyment, and the school's success in narrowing the attainment gap suffered by disadvantaged pupils. All these would sit alongside raw exam results.
The plan to use them was first disclosed by The TES in September. When the Government officially unveiled the idea a month later, it admitted that league tables and Ofsted reports were failing to provide a true representation of school performance.
But the idea got a cool initial reception from the unions.
John Dunford, ASCL's general secretary, said school performance was too complex to be reduced to a single grade. However, he said the idea had potential to provide a more accurate picture if school intake was taken into account.
"The devil will be in the detail," he said at the time. "It must replace league tables, not be in addition to them."
Ministers said they would continue to publish the information used to compile school league tables. It would be up to the media to decide whether to use the information on report cards instead.
This stance has angered the NUT.
"Had the school report card been proposed as a replacement for school performance tables and as a trigger for an independent review of the current overlapping and contradictory school accountability system, then it would have been in contention for serious consideration," the union says.
"As it is, it appears to be both another burden and also a variant of the current school performance tables."
Like the ASCL, the NUT also rejects the idea that the data on the cards can be aggregated to a single numerical score or letter rating.
"Accurately reflecting information about the performance of schools across a wide range of activities is a very complex matter," it has warned the Government.
"Simplifying this complexity into a single score will inevitably only provide a very crude measure of performance."
Ministers have asked their panel of experts considering future assessment arrangements to advise on the potential and design of the report card.
On the panel, alongside Sir Tim Brighouse, the former commissioner for London schools, and Sir Jim Rose, leader of an independent review of the primary curriculum, are Maurice Smith, a former chief inspector of schools, and headteachers Yasmin Bevan and Gill Mills.