The schools watchdog has admitted one of its inspectors made a "mistake" in not speaking to pupils at a Muslim girls' school because the children were celebrating a religious festival.
Ofsted said it had taken "appropriate action" against the inspector following the unannounced emergency inspection at the Zakaria Muslim Girls' High School in Batley, West Yorkshire.
The inspector said he could speak only to senior managers, including the head teacher, at the independent school, which has 147 pupils aged between 11 and 16 and is run by members of the Deobandi sect, which teaches an orthodox view of Islam.
The report, dated in October last year, said: "It was not possible to talk to students during this visit as they and the staff were celebrating the festival of Eid."
The school, which was established in 1982 and costs up to £1,300 a year for day pupils, was initially found to have met Government requirements for safeguarding students on issues such as radicalisation and female genital mutilation.
A second inspection was carried out in December following the error, an Ofsted spokeswoman said.
She said: "We can confirm that an inspector failed to speak with students during an inspection of Zakaria Muslim Girls High School in October 2015.
"This was a mistake and we have taken appropriate action regarding the inspector.
"We carried out another inspection of the school in December 2015 and we are in discussion with the Department for Education about further monitoring of this school."
Sky News reported that the school is one of three facing further action following an investigation it has carried out into the Deobandi sect.
A Department for Education spokesman said: "As soon as concerns were raised we launched urgent investigations and while these are ongoing, it would be inappropriate to comment further.
"Extremism has no place in our society and it is vital all schools are providing a high quality, broad and balanced curriculum.
"Where schools are not doing this and are focusing on ideological indoctrination instead, we will not hesitate to take action including closing the school or working with the police if necessary."