Supply teaching shocked Alex Dolan. The teacher-turned-reporter says it was her experiences on supply in London comprehensives that led her to make Dispatches: Undercover Teacher, the Channel 4 documentary in which she secretly filmed the disruption and misbehaviour that she regularly witnessed.
"Supply teaching opened my eyes to how many teachers do such a great job in difficult circumstances," she said.
"We all know supply teachers have a hard time. Kids see them as opponents.
But the system is failing the kids. They're not getting the education they should."
Ms Dolan had taught science for two years before leaving the profession to pursue a career in the media. Between short-term television contracts, she took supply teaching work to supplement her income.
She insists that the programme, which was filmed over three months, was representative of her observations, and she was personally involved in its editing.
The 30-year-old took a hidden camera into 16 schools around the country.
Footage from two London comprehensives and two in Leeds were eventually used in the programme.
"We wanted to be fair to the schools," she said. "Each school had individual issues that we wanted to make points about, so we went through it with a fine-tooth comb.
"I was so absorbed in the teaching. It was easy to forget there was a camera in my bag. I didn't make people look like they said anything they didn't say.
"Some of the things I saw had my heart going, like I was a pupil being told off."
The day the programme was broadcast, last Thursday, was one Ms Dolan describes as memorable: walking through Tavistock Square, in London, she witnessed the number 30 bus explode.
Despite the terrorist attacks in London that day, her programme was watched by 1.7 million viewers. It has since generated significant notoriety for her. Even so, she has not ruled out a return to the classroom.
"There were moments, during the investigation where suddenly I held the kids' attention. You could have heard a pin drop, they were so enthralled.
It's a wonderful feeling.
"I do think I will be a full-time teacher again. But not now. There are easier ways of earning a living."
But the final word must surely be left to the pupils Ms Dolan worked with.
Patrick McCarthy, 13, a pupil at St Aloysius' college, in Islington, north London, said: "She wasn't the best of teachers. She said our teachers were strict, but if anyone done anything wrong, she was more strict than them.
"She wasn't really teaching in her lessons. But it's probably difficult to teach and record at the same time."