By Hazel Shearing
Education correspondent
Most of the outstanding schools successful England inspected since 2020 person been downgraded, according to a study from the schools watchdog.
Some of them had not been looked astatine for 15 years and galore would person experienced "significant change" specified arsenic caller caput teachers, Ofsted said.
But the National Education Union (NEU) said Ofsted's findings were "frequently unreliable".
The Department for Education said astir schools remained bully oregon outstanding.
Between 2012 and 2020, schools judged outstanding were revisited lone if circumstantial concerns were raised.
Ofsted said 80% of outstanding schools it had revisited past twelvemonth had been downgraded - 308 superior and secondary schools.
Most were bumped down to "good" - but 17% were told they needed betterment and 4% were "inadequate".
Ofsted said it had prioritised schools that had gone the longest without inspection, erstwhile it had been deciding which schools to look astatine past year.
On average, the schools it visited had not been inspected for 13 years - but immoderate had gone arsenic agelong arsenic 15 without an inspection.
'Ill-informed inspection'
Ofsted main inspector Amanda Spielman said the exemption had "deprived parents of up-to-date information" and meant schools had been without "constructive challenge".
"The exemption was a argumentation founded connected the anticipation that precocious standards, erstwhile achieved, would ne'er driblet and that state from inspection mightiness thrust them adjacent higher," she said.
"These outcomes amusement that removing a schoolhouse from scrutiny does not marque it better."
NEU associated wide caput Kevin Courtney said Ofsted's findings were "frequently unreliable and invalid".
"Far from demonstrating the worth of Ofsted, this study shows that the inspectorate makes nary worldly affirmative quality to schools," helium said.
Ofsted was "driving bully teachers away" and "punitive and ill-informed inspection" slowing down improvements.
'Rapidly improved'
The Institute for Fiscal Studies said an summation successful backing for the adjacent 2 years, in past week's Autumn Statement, would instrumentality real-terms per pupil spending lone to its 2010 level.
A Department for Education spokesperson said the authorities had "rapidly improved schoolhouse standards, acknowledgment to the tireless efforts of schoolhouse leaders".
"Today, 87% of schools are present rated arsenic bully oregon outstanding - up from 68% successful 2010," helium said.
Ofsted aimed to inspect each schoolhouse by the summertime of 2025, the spokesperson added.