"Filming or recording on these premises is only allowed with written permission from the headteacher." So reads the sign in the entrance hall of St Aloysius' college, in Islington, north London.
The 1,000-pupil boys' school was one of four comprehensives featured in the documentary Dispatches: Undercover Teacher, shown on Channel 4 last week.
Alex Dolan, a reporter and teacher, spent several days on supply at each school, recording events on a hidden camera.
Footage recorded at Highbury Grove, also in Islington, showed a pupil wielding a fire extinguisher at classmates. Scenes in two Leeds comprehensives, John Smeaton and Intake, showed similar chaos.
By contrast, St Aloysius' was presented as a zero-tolerance environment, with a regime one member of staff was recorded comparing with Guantanamo Bay.
Danny Coyle, deputy head, condemned Ms Dolan's technique: "She was approachable, chatty and personable, full of questions. Obviously we know why now. It's a complete betrayal of trust.
"We thought we were portrayed as quite brutal. But it's a happy school. The staff are happy and the kids are happy."
Two members of staff have been summoned for formal talks with the head after being shown calling a class "total scum" after a vandalism incident, and telling a boy to "bugger off home".
But the school is unapologetic about its emphasis on discipline. Exams are over and the sun shines outside, but there are no Hollywood videos or nature walks in evidence. Deputy heads patrol quiet corridors, looking in on lessons to monitor behaviour.
Strolling into a history class, Mr Coyle suddenly clicks his fingers. A small boy removes his schoolbag from his desk, and places it on the floor.
"Sorry, sir," he murmurs. On his desk, a textbook about medieval views of the afterlife reads "Welcome to Hell".
"One parent called up and said she was very proud of the programme," said Mr Coyle. "She said a strict school with good values reminds her of being back in Ghana in the 1950s."
One of the most ardent supporters of the school's discipline policy is art teacher Denis Doherty. He resents that his "Guantanamo Bay" remark, used in reference to misbehaving pupils forced to work in silence in the school hall, was taken out of context.
"It was just a joke," he said. "But it was edited so that she asked me what I think of school-discipline policy, and I say it's like Guantanamo Bay.
I've been advocating firm discipline for years. If sanctions are there and you can act on them, you have a much better working situation in school."
Mr Doherty has asked his union to help him acquire Ms Dolan's unedited footage from Channel 4.
Union officials have also criticised the programme. In Leeds, Jack Jackson, a representative for the NASUWT, the second-largest teachers' union, has been contacted by concerned members.
"Alex Dolan talked to people in the privacy of the staffroom and, of all places, the ladies' loo," he said. "It's a breach of confidence.
"I hope anything said will be considered to have been in private circumstances."
But the Leeds branch of the National Union of Teachers has received no complaints. The local representative said he fell asleep halfway through the programme.
A parent of a 13-year-old pupil at Highbury Grove, who asked to remain anonymous, insists that the film did not give a representative picture of daily school life.
"It wouldn't be an interesting programme unless it showed something a little bit shocking and tabloidy," he said. "It was cheap television - a cheap shot."