Ofsted should scrap its “outstanding” grade, and instead simply conclude whether or not a school meets required standards of education, a heads’ union has claimed.

The radical proposal has been drawn up by the Association of School and College Leaders’ (ASCL), as part of its vision of a slimmed-down inspectorate designed to “reduce the unhealthy extent to which the threat of inspection dominates many schools’ work”.

Ahead of the union’s annual conference this month, general secretary Brian Lightman has written a discussion paper on how Ofsted should operate in the future.

While schools are at present placed in one of four categories - outstanding, good, “requires improvement” or inadequate - Mr Lightman (pictured) argues that forcing inspectors to simply decide whether a school meets required standards or not would leave it free to experiment without fear of inspection.

“Ofsted’s role should be one of quality assurance - to determine whether schools meet the expected standard or not,” he writes.

“School and college leaders are not confident that the current system of grading schools is consistently applied. Decisions at the grade-thresholds are often unreliable.

“A system which determined only whether schools meet the standard or not would free schools to innovate without having to conform to a set of externally imposed, often narrowly conceived criteria.

“It would reduce the unhealthy extent to which the threat of inspection dominates many schools’ work and makes teachers afraid to try new approaches in case they do not meet with ‘Ofsted approval’.”

ASCL’s intervention in the debate about the future of Ofsted is the first of three expected in the coming days.

Two right-leaning think tanks, Civitas and Policy Exchange, are expected to publish reports on the current school regime inspection.

The debate comes amid rising tensions between the inspectorate and the Department for Education, centering on Michael Gove’s decision not to renew the contract of Ofsted’s chair, Baroness Sally Morgan.

In his discussion paper, which will be finalised ahead of ASCL’s conference in Birmingham, Mr Lightman argues that there is a “number of significant problems” with Ofsted’s current approach.

These include “inconsistency in inspection judgements” and “confusion about what Ofsted is looking for”.

Mr Lightman also suggests that Ofsted should not renew the contracts it has with private providers of additional inspectors. Inspection teams should, in future, consist of Her Majesty’s Inspectors assisted by serving or recently retired school leaders working directly for Ofsted, he adds.

In order to combat the “culture of fear around inspection which hampers innovation and sensible risk-taking”, Mr Lightman calls for concerns about schools flagged up by Ofsted’s data-led risk assessments to initially be addressed by an informal investigation by an inspector, rather than a full inspection.

Newly-appointed heads in challenging schools, he adds, should be given time to make improvements in order to lesson the perceived threat of “career suicide” by taking on a challenging school.

The proposals will be debated further by ASCL members ahead of its annual conference, which begins on 21 March. Mr Gove and Ofsted’s chief inspector, Sir Michael Wilshaw, are scheduled to appear.