Ofsted has carried out its first "dawn raid" on a school as the watchdog pushes ahead with no-notice inspections, despite fierce opposition from teachers' leaders.
The headteacher of the inspected school has backed the process, saying "it is a good way to do it".
But teachers' unions believe the idea shows a lack of trust in the profession and this week told an influential parliamentary committee that it was a "nonsense".
Inspectors' leaders are also concerned that the lack of notice will make it difficult for them to meet school leaders if they are off the premises at the time of the visit.
But Louise Whitgreave, the first head to receive an unannounced visit from Ofsted, told The TES it was "quite a positive experience".
The head said she panicked when inspectors arrived at St Andrew's C of E Primary in Nesscliffe, Shropshire, at 8.30am in the middle of a snowstorm last month.
"You are like a rabbit in the headlights at first, but after half an hour everything calmed down," she said.
"They worked around us and were flexible. They didn't just come in with a clipboard. Beforehand, I didn't like the idea of no-notice inspections, and nor did my staff - we thought it would be terrible.
"But it is a good way to do it because they have seen us warts-and-all on a normal working day."
The Shropshire primary, which was judged to be good with outstanding features, was told by inspectors that it was the first school to receive a no-notice inspection as part of Ofsted's pilot scheme.
It is understood that at least one other school has since been inspected under the new regime.
The prospect of it being used in all schools looks increasingly likely: Ofsted stated last week in a new document - How We Inspect - that "no notice should be the aim of all inspection and regulation".
The idea led Mick Brookes, general secretary of the National Association of Head Teachers, to tell the Commons schools select committee that it was a "nonsense" and the logistics had not been thought through.
But Mrs Whitgreave said the existing system, in which schools might be told on a Thursday that inspectors were coming on the following Tuesday, led to staff "running around like headless chickens".
Teachers would spend all weekend in school, adjusting displays and printing off so many school policies that inspectors would end up "high on printing ink", she said.
She added: "It means you don't have to prepare this show for inspectors. It will definitely make the time leading up to inspections less stressful."
How We Inspect sets out the idea of "no notice" as a basic principle for all Ofsted inspections, but says this would be subject to practicalities such as security clearance and the need to interview particular members of staff.
The document follows the watchdog's publication of research earlier this month that suggests two-thirds of parents support no-notice inspections.
An Ofsted spokesman said there would be a full evaluation of the pilot scheme before "any firm decisions are made".
John Dunford, general secretary of the Association of School and College Leaders, said: "It will be a sad day for education if Ofsted moves to no- notice inspections. They come from the school of thought that inspection is something that should be done to schools because they are doing something wrong."
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