The practice of grouping classes by ability has long had strong backing from the top. Ofsted, the education secretary, the prime minister and their Labour predecessors have all encouraged schools to use setting in more lessons.
But, despite their rhetoric, Conservative ministers have quietly dropped a pledge to enforce setting by ability, TES can reveal. It has also emerged that neither the government nor Ofsted know how much setting and streaming is used in schools because they do not collect accurate data.
Michael Gove first set out his stall on the matter in a schools Green Paper in 2007, when the Conservatives were in opposition. It argued that "each pupil should be given the opportunity to learn in accordance with their particular aptitude and ability". "We believe that setting by ability is the only solution to achieving this ambition," it added.
Mr Gove pledged that he would "alter guidance to Ofsted to ensure that schools - particularly those not performing at high levels - set all academic subjects by ability".
The paper was endorsed by David Cameron, who had previously declared himself "passionate about the importance of setting by ability" and had promised "a 'grammar stream' in every subject in every school".
Last September, Ofsted chief inspector Sir Michael Wilshaw appeared to support the call, warning that some students were being held back by "the curse of mixed-ability classes without mixed-ability teaching", adding that such teaching was "hugely difficult" to achieve.
But the government has now said that it does not advocate setting. "It is for schools to decide how best to organise teaching - including whether to group and set pupils by ability - as they know exactly what their students need," a spokesman said.
And Ofsted says it "doesn't have a view on whether setting or streaming is a good idea or not". A spokeswoman for the inspectorate also revealed that Conservative ministers had not asked Ofsted to enforce setting.
The Department for Education was unable to produce statistics on how many students are set or streamed. Ofsted produced limited data based on lessons it had inspected (see panel, right), but stressed that "there is no way of using this data to draw out national conclusions in any way".
Heads' leaders were critical of the perceived pressure to group children by ability.
"What matters is pupil achievement, and how schools manage that is a professional decision to be taken locally," said Brian Lightman, general secretary of the Association of School and College Leaders.
In comments accompanying Ofsted's figures, Sir Michael noted that, since 2005, its inspections have not involved observing all teachers in a school. Lessons that were seen were not "necessarily representative" of the school or system as a whole, he said.
Comparisons over time were also difficult because, from 2003 onwards, the watchdog stopped distinguishing between streaming and setting in its figures and amalgamated the two categories, Sir Michael said.
These problems were acknowledged in 2009 when Christine Gilbert, Sir Michael's predecessor, admitted that the lack of "comprehensive data" on school setting was "of some concern". The former Department for Children, Schools and Families admitted that there was not an "accurate picture" and said Ofsted would collect more information.
But Sir Michael's comments suggest that, despite the rhetoric, little has changed. "It is not possible to deduce from inspection data the proportions of pupils nationally who are taught in settedstreamed classes or in mixed-ability groups," the chief inspector said.
"This is because Ofsted does not collect data on the number of pupils who are taught in this way."
Percentage of secondary school lessons (excluding PE) observed by Ofsted that were set or streamed:
45% - 1997-98
44% - 2001-02
46% - 2005-06
45% - 2010-11.