The country's largest Jewish school was the subject of an unannounced inspection from Ofsted earlier this month, the JC has learned.
But neither JFS nor the inspection service would divulge the reason for the two-day visit.
Ofsted would say only that the inspection had been prompted by "concerns" it had received.
A spokesman for the school said that it was not at liberty to disclose the purpose of the inspection until the findings were published, which would be likely in the first half of September.
Schools are normally alerted in advance of a regular visit from the inspection service.
But Ofsted recently announced the increased use of surprise inspections following its investigation into alleged Islamic extremism in several state schools in Birmingham.
One veteran Jewish educationist, who did not wish to be named, said the inspection "does seem very strange" and speculated that there could be a knock-on effect from the Birmingham inquiry.
"None of the Birmingham schools are faith schools," he said. "What is a problem for faith schools is that we are being tarred with the same brush."
In its last two regular Ofsted inspections, in 2006 and 2009, JFS was ranked an outstanding school - and it has continued to perform well academically.
Previously, the policy had been for outstanding schools to be inspected only every five years and former Education Secretary Michael Gove had favoured a lighter inspection regime for the best schools.
Meanwhile, a fresh report into the Birmingham schools affair was published this week by Education Commissioner Peter Clarke, who found evidence of a concerted attempt to introduce "an aggressive and intolerant Islamic ethos" into a number of non-faith state schools.
Apart from references to hardline preachers and homophobic views, on two occasions "offensive images that can only be interpreted as anti-Israeli" were posted on a web discussion group.
In one school, Mr Clarke reported, "a three year-old in nursery said that his family was poor because, 'All the Jews and Zionists have all the money'."