Ofsted chief inspector Amanda Spielman has called for the inspection exemption on "outstanding" schools to be scrapped, as new figures show that more than 80 per cent of top-rated schools visited by inspectors this year were no longer "outstanding".
The watchdog can inspect "outstanding" schools where it has concerns over standards or safeguarding but they are otherwise exempt from routine reinspection.
Figures published today show that only 16 per cent of the 305 "outstanding" primary and secondary schools inspected this academic year retained their top Ofsted rating.
And almost a third of previously "outstanding" schools were not rated as "good" by inspectors.
Ministers: 'Inspect 10 per cent of "outstanding" schools'
This academic year, Ofsted has "substantially increased" the number of exempt "outstanding" schools that it inspects.
Ofsted fears over 'outstanding' schools
The inspectorate said this was partly down to concerns about some schools having been exempt for so long that parents can no longer have confidence in their "outstanding" grade.
Ms Spielman said: “Today’s figures are not particularly surprising, but they should still set alarm bells ringing.
“The fact that 'outstanding' schools are largely exempt from inspection leaves us with real gaps in our knowledge about the quality of education and safeguarding in these schools.
"Some of them have not been inspected for over a decade, and when our inspectors go back in, they sometimes find standards have significantly declined.
“We believe most schools judged 'outstanding' are still doing outstanding work. But for the 'outstanding' grade to be properly meaningful and a genuine beacon of excellence, the exemption should be lifted and Ofsted resourced to routinely inspect these schools.”
Between 1 September 2018 and 31 March 2019, Ofsted inspected 305 "outstanding" primary and secondary schools, which represents 8 per cent of all exempt schools and is more than double the number inspected during the 2017-18 academic year.
Last year schools standards minister Nick Gibb said Ofsted should inspect 10 per cent of "outstanding" schools after the inspectorate raised concerns about the exemption.
Ofsted said that, given that most of these inspections were carried out because the school’s performance appeared to be declining, it was not surprising for a large proportion to lose the top grading.
However, only 49 of 305 (16 per cent) exempt schools inspected so far this academic year have remained "outstanding", compared with 49 out of 150 (33 per cent) inspected between 1 September 2017 and 31 August 2018.
Among those schools that lost the top rating, 166 were judged to be "good" (54 per cent), while 76 were found to "require improvement" (25 per cent) and 14 were rated "inadequate" (5 per cent).
In keeping with the past two years, more primary than secondary schools lost their "outstanding" grade.
Only 31 of 239 exempt primary schools remained "outstanding" this year (13 per cent), while 18 out of 66 secondary schools kept the top grade (27 per cent).
Today’s figures also show a slight drop in the overall proportion of schools judged good or outstanding at their most recent inspection; from 87 per cent in August 2017, down to 85 per cent at the end of March 2019. Outcomes continue to be higher for primary schools (87 per cent "good" or "outstanding") than secondary schools (75 per cent).