“Before, you could plan. Now you can’t even book supply staff with any certainty two days before,” he sighs. “You’re lucky if you get a supply teacher at all, let alone a specialist.”

With at least 10 slots to fill through vacancies, illness or training on a typical day at George Green’s school in east London’s Isle of Dogs, the cover co-ordinator can spend an hour each night trying to draw up the timetable, only to rip it up the next morning.

“A couple of years ago, 90 per cent of supply teachers would be specialists,” he said. “Now less than half are, even long-term.” Some can cover for two or three months in subjects they were not trained to teach.

The crunch comes when places cannot be filled. Other supply teachers at the school can take over - unlike permanent staff, there is no contract clause to stop them. Failing that, senior managers step in. “We don’t ask people here to cover for more than three days and we’ve been lucky enough to get long-term supply cover,” says head Kenny Frederick. Nevertheless, staff voted solidly for the no-cover actin, saying the situation is having an impact on them and students.

“We have to organise the lessons that supply teachers are teaching,” English teacher Claire Trevaskus said. “We’ve got enough to cope with without other people’s workload.”

That could end soon. National Union of Teachers’ guidelines say members should not plan supply staff’s lessons or take split classes.

But other issues affect them. Head of drama Willie Deighan says pupil behaviour is “not getting any better”. He abstained in the ballot - “I’m not sure action is appropriate,” he says - but echoes his colleagues’ concerns.

“Supply teachers come in, kids give them a hard time and they send them to us to help. It takes up a lot of time and in terms of stress it’s an enormous drain.” But he is loath to disrupt pupils’ education further - and there’s the rub. Teachers at George Green’s are furious at David Blunkett’s suggestion that they don’t care about their pupils.

For many, the no-cover action is a visible way of expressing wider frustrations. Head of English Sarah Davies sums it up: “Our profession is being undermined daily by the media, and the present Government is no different from the last one.”