Tristram Jones-Parry, 57, the retiring head of Westminster school, was excluded from the state system because he did not have qualified teacher status.
The Independent Schools Council (ISC) said that an agreement was negotiated four years ago allowing private school staff to be fast-tracked to qualified teacher status (QTS), but this route was closed in September without any official reminders.
Dick Davison, the council's spokesman said: "It was agreed following negotiations that those who had begun teaching before 1974, or before 1989 if they were teaching shortage subjects such as maths, science and modern foreign languages, should follow a straightforward assessment and gain QTS automatically."
In a separate case last month, the retired American academic Professor David Wolfe, 65, was barred from teaching physics at a Buckinghamshire school, even though he has a PhD in the subject and 30 years' experience lecturing in the United States.
Margaret Tulloch of the Campaign for State Education said she agreed with the decision not to allow them to teach. "I cannot imagine that these two people would have had difficulty gaining QTS," she said. "But, it has been central to our campaign over the years that, for the sake of pupils, one of their entitlements in getting a decent education should be that they are taught by qualified teachers."
Professor Ted Wragg, emeritus professor of education at Exeter University and TES columnist, said: "Some people remain ideologically opposed to training but we cannot go back to what it was like in the 1970s when it was enough to have a degree to go into teaching.
"Even if you have been teaching for 30 years it does not mean you have been teaching well. Even in exceptional circumstances you could not give QTS automatically."
But Professor Adrian Smith, principal of Queen Mary College, University of London, said he thought the decision to bar the two men was "totally absurd". "One would have thought some kind of wit could be used to get around the situation," he said.
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