Mystified by the deep rumblings of teenage traditionalism, he invoked the memory of R H Tawney, the socialist thinker and author of Equality, who was a pupil at the end of the last century.
"Tawney would have been very disappointed," he said. "I don't think we have much radicalism at present."
About half of the school's 500 boys took part in the boycott of a service to commemorate the 200th anniversary of the birth of Thomas Arnold, former headmaster and leading educational reformer. They were led by a gang of sixth-formers who had scattered posters around the school - founded in 1567 - following the appointment of Louise Woolcock, 17, as head girl in addition to Huw Brown as head boy.
The "fliers" contained the slogan "We are not sexists, we are traditionalists" and the more complex "Girls don't play rugby football, boys don't play netball. Please don't confuse us."
Mr Montgomerie said that parts of the school, including School House, the seat of the rebellion, were being refurbished and the disruption had played its part in the protest. In a reference to another famous, although fictional pupil, Tom Brown, he said: "Tom Brown wouldn't recognise it. Boys don't like to have their way of life disrupted, they always think it will be for the worse."
The protest campaign follows earlier episodes of graffiti daubing "Girls Out" when Michael Mavor, the head, was appointed five years ago and committed the school to full co-education status.
Sixth-form girls have been present for 20 years in the school with boarding fees of pounds 12,270. Charlie Morrell, of the lower sixth, was put forward by the school's authorities as a spokesman for the rebels. He said they objected to Ms Woolcock's appointment as she had only been at the school for three terms and so did not have enough experience of the school's traditions.
Boys wanted to wait a further two years, by which time there would have been girls who would have been all through the school rather than just in the sixth form.
Like most other service absentees, Mr Morrell had been "gated" for 24 hours (not allowed to leave the grounds) in punishment.
"Louise has only been here nine months," he said. "We feel it would be more acceptable to have a girl that had been here five years. It's not the principle that's wrong, it's just been done in the wrong way at the wrong time." A more radical view was expressed by James Roper, 18, who was due to leave the school next week after sitting A-level exams in English, French and history.
"It's not political correctness," he said, ignoring an instruction not to talk to the press. "It's just the fact that Mr Mavor came to make the school co-ed and it has now reached its peak. The only reason he has done it is for PR."
A member of the first team in rugby, football and cricket, Mr Roper accused the head of sacrificing school sport in pursuit of academic achievements and in favour of introducing girls.
"Mr Montgomerie told us in the pub that if we said anything else we would have to do our exams at another school," he added. Rugby's pupils over the age of 18 are allowed to visit pubs on Saturday nights.
Ms Woolcock was suitably cool in a succession of radio, television and newspaper interviews. Studying biology, chemistry and maths and hoping to follow Clare, her sister and a former Rugby pupil, to study medicine, she was confident that the school would settle down in couple of days.
"It's the Nineties after all. By next term they will be used to it," she said.
"Boys being boys, they will argue about anything."
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