It is every teacher’s worst nightmare: an upcoming inspection, coupled with a rowdy, misbehaving class, and a grievous lack of preparation.

The solution, though, is simple. Merely lock any miscreants in the school cupboard and bus in a coachload of meek, well-behaved children to fill the classes instead.

These are the latest career-enhancement tips from Teachers, the long-running Channel 4 series.

The cast have just begun filming an eight-episode fourth series, to be shown in the run-up to Christmas, and a Christmas special. It is the first Channel 4 drama to have had a fourth series commissioned.

The new series will show the staff of Summerdown submitting to their first Office for Standards in Education inspection, despite all teachers apart from the head having forgotten that inspectors were due.

Staff also have to contend with a number of school-wide education campaigns, such as Obesity Awareness Week, managing to embarrass both themselves and their pupils in the process.

Meanwhile Bob, the balding English teacher, embarks on a midlife crisis, complete with new hairpiece, scooter and fiancee.

Jane Fallon, the executive producer, believes that such cringeworthy classroom moments explain the series’ appeal in real-life staffrooms.

“I’m surprised at the number of teachers who watch,” she said.

“But I’ve met teachers who say that many of the characters are believable, such as the slightly arsey school secretary.

“We based the teachers who climb up on the roof to smoke on someone who told us about a no-smoking rule at their school. But we heighten situations in a way that would never happen in a real school. It’s nice escapism for them (teachers).

“Our teachers say things that teachers could never realistically say to a child. The events are grounded in reality, but they always go completely awry.”

In the forthcoming series the greatest potential for misunderstanding and mishap comes from the recent merger between Summerdown and a neighbouring comprehensive.

“Our lot are used to being top dog,” said Ms Fallon. “But they have moved to someone else’s school. They have to compete for their own jobs. So, this time round, they’re a little bit on the defensive.”

Among the new staff from the neighbouring school are Ben, a hypochondriac atheist teaching religious education, and Damien, a cynical food technology teacher. However, most staffroom attention is focused on Ewan, the young, good-looking head of English.

“He’s very sweet and naive,” said Ms Fallon.

“He wakes up in the morning with a woman next to him and has no idea how it happened. The other teachers are quite grumpy about this.

“We’ve brought in new characters, because we don’t want the plot to stagnate. We have to keep it fresh and interesting.”