It is more than a decade since the term "assessment for learning" became common currency in schools, but the actual practice has yet to take hold, according to an academic who popularised the idea.
Paul Black co-authored Inside the Black Box, the 1998 book that sold tens of thousands of copies and introduced a generation of teachers to the notion of using tests as an aid to help pupils improve, instead of just grading them.
Now he admits that assessment for learning (AFL) "isn't happening" in a "very large number of classrooms".
Professor Black, of King's College London, suggested a major reason was the failure of many teachers to properly grasp the concept.
Some wrongly assumed AFL was about summative assessment - measuring pupil progress and meeting targets - an idea encouraged by some Government initiatives, rather than Black's idea of formative assessment.
Other teachers thought it was just about good teaching and that they were doing it already.
"Sadly, they are not," Professor Black told a Cambridge Assessment conference. "It is not that they are being dishonest. They think they are interacting with their pupils and involving them in discussion.
"But an observer in their classroom could say to them: 'You thought you were only talking 50 per cent of the time. But I am sorry to tell you you were talking 80 per cent of the time'."
Professor Black said there were also teachers who thought AFL was "too risky" and "too radical" because it involved changing their role and interacting with pupils who could be poorly behaved and difficult to motivate.
Others believed they had no time for AFL because they had to cover the syllabus and "train pupils for the test".
"The evidence is that those who have spent time doing it get better test results than those who drill pupils for the test," Professor Black said. "But that evidence won't convince a teacher who is risking their reputation, their career, if they embark on a new trajectory of teaching."