Twenty-four hours after being banned from teaching for secretly filming the anarchic classrooms of the schools in which she worked as a supply teacher, Alex Dolan is still fuming. “I wanted to make this programme so the public could see what is really going on in our schools,” she says. “I am very disappointed that the powers that be have chosen to discipline me rather than face up to and deal with the serious problems I uncovered.

“What about disciplining the teachers I filmed calling children ‘scum’, or the ones I exposed who were hoodwinking Ofsted inspectors about the state of their school? What about them?”

It’s been four years since Dolan, 34, who has a degree in marine biology from New-castle University and trained as a science teacher, went undercover into four schools: two in London and two in Leeds. She was armed with two hidden cameras – one in a bum bag, the other in a box file.

“Some of the things I captured on film I still find jaw-dropping,” she says. Among the many disturbing episodes was one in which, working as a supply teacher, she was threatened by a female pupil. Asked by Dolan to stop playing music on her mobile phone, the teenager squared her shoulders aggressively and said: “Don’t make me hurt you. I swear to God I will do it.”

While undercover in another comprehensive, she spoke to a 15-year-old girl who was writing to Tony Blair to plead for a permanent science teacher because she had had 26 supply teachers in six months. “Our GCSEs are at risk,” the girl told the prime minister. In a third school, in Leeds, Dolan recorded the deception of a group of Ofsted inspectors: “I walked into one classroom and found a male science teacher with six of the naughtiest children eating cake at the back of the class. He told me, ‘I’ve taken them out of the laboratory while the inspectors go round’.”

The footage she recorded was shown as a Channel 4 Dispatches programme in 2005 and started a debate about the state of Britain’s classrooms. Last Wednesday the General Teaching Council (GTC) ruled that Dolan was guilty of breaching pupils’ trust and abusing her position. After an eight-day hearing in Birmingham, the council banned her from the classroom for a year, rejecting her claim that the secret filming was in the public interest.

“I did it for the children,” she says defiantly. “Not for myself at all. There are so many children out there not getting a decent education. Parents and pupils have a right to know what is going on, and I felt that something should be done about it.”

It’s a view shared by Chris Woodhead, the former chief inspector of schools, now a Sunday Times columnist, who gave evidence for Dolan, telling the hearing: “If anyone should be in the dock, it should be the people responsible for the education of those children.”

Undercover Teacher was aired on July 7, 2005, the day the terrorists attacked the London underground. Dolan, who was in Tavistock Square when the bus was blown up, killing 13 people – “I ran towards Rus-sell Square without looking back,” she recalls – wonders whether it would have made even more of an impact if it had been shown on a less eventful day.

Her aim was always, she says, to get things changed. The GTC suggested she was simply trying to further her burgeoning career as a television presenter, but she fiercely denies that: “I went into teaching straight after my degree. I loved it.”

Children’s faces were blurred to protect them and before the film was screened it was shown to parents, some of whom were so infuriated that they took legal action to try to stop the documentary. A High Court judge ruled it was in the public interest that it be shown.

Dolan’s first teaching job was at Lady Margaret in Parsons Green, south London, one of Britain’s top state schools, where Nigella Lawson’s sister had been head girl. When Dolan, a science teacher at the school, invited the journalist John Diamond, Nigella Lawson’s late first husband, to come to talk to her GCSE class about the link between smoking and cancer, she unwittingly took the first step towards a career change. “By then he had no tongue because of his cancer, but the kids tuned into him – they asked him the most personal questions.”

Encouraged by Diamond’s example, she realised she wanted to try her hand at television and a year later left teaching to be a researcher on Tomorrow’s World. In between TV contracts she turned to supply teaching to pay the bills and it was in the classrooms of inner-city comprehensives that her eyes were opened to the reality of some of our most troubled schools.

“There were things going on that shocked me beyond belief. I had a class of GCSE kids in a north-London school. They hadn’t done their coursework. The head of department just said, ‘Write something on the board and get them to copy it out’.” She knew straightaway she wanted to expose such cheating and kept a diary, which she sent to various newspapers. Then she hit on the idea of an undercover film. From January to April 2005 she surreptitiously recorded, with backing from Channel 4. “No one knew what I was doing; not even my mum,” she recalls.

With her camera running, she captured not just the misdeeds of badly behaved children but also episodes of teachers abusing pupils. “One vulnerable child was told to ‘bugger off’,” she recalls. Others were described, after an incident in which school property was damaged, as “scum”.

In an Islington comprehensive she was advised to keep her classroom door locked to stop the hordes marauding through the corridors disrupting her lesson, and on one occasion stood between two boys, one armed with a chair, to break up a fight. “I kept asking people: is it just because I am a supply teacher? And they would say, ‘No, this is how it is’.”

She sometimes wonders what happened to the pupils she met: “I often think of those children and wonder if they had any decent education and what their lives are like. I’d like to find out. Four years on, I still feel passionately about this.”

Times recommends

• Students abandon the drink for gyms and juice
• Ofsted: third of Baby P-style reviews inadequate
• Top boys' school picks first woman head