Carefully, deliberately, Ed Balls takes off his jacket and places it over the back of the chair. Then he rolls his shirt sleeves up above his elbows and glances at the classroom clock.

At exactly 2.10pm, he marches to the door and lets in the waiting pupils. “Good morning!” he says cheerily. There is a slight pause. “Ah, um, good afternoon.”

Normally, a beginning like that would guarantee a supply teacher a lesson’s worth of uninterrupted classroom hell. But, on this occasion, the pupils walk swiftly to their seats.

“Right,” their teacher says. “My name is Ed Balls, and I’m the person who’s in charge of all the schools in the country. But today, I’m your teacher.”

The Schools Secretary is delivering a one-off English lesson to Year 7 pupils at Westminster Academy in inner London. The lesson is intended to promote Teach First, the scheme that encourages high-flying graduates to spend two years teaching in inner-city classrooms.

Mr Balls begins by asking pupils to look at six photographs on a handout, and name the characteristic shared by those pictured. He points to the first photograph, of Martin Luther King, and a girl volunteers that “he tried to stop racism”.

Pupils then correctly identify the Dalai Lama and Gandhi. Next, Mr Balls points to the picture of Bob Geldof. There is utter silence. “I can’t believe this,” he says. “How old are you guys? Did you not know the Boomtown Rats in the mid-1980s?” More silence. “Okay. Who can tell me one thing about Bob Geldof?”

Tentatively, one boy puts his hand up. “Yes?” says Mr Balls. “He was in the 1980s,” the boy says.

His teacher tries a different tack. “What happened to Martin Luther King?” Another boy puts up his hand: “I think, when he went to the beach, he was beaten up.”

Mr Balls is slightly nonplussed. “Well, that may have happened. Later he was murdered in the Lorraine hotel in ... a city in America I’m going to have to think about.”

He moves over to the interactive whiteboard, and plays a clip from the film Gandhi, in which a young Mahatma persuades his followers that non-violent protest is more effective than direct confrontation.

He then puts on a video of Martin Luther King delivering his “I have a dream” oration. “What was Martin Luther King saying there?” he asks. A girl raises her hand: “He has a dream?”

Eventually, the pupils grasp Mr Balls’ point: that non-violent protest can be a successful means of effecting political change. He then commissions the pupils to write a persuasive speech, campaigning for change in their own school.

Some pupils take to this task immediately: “I have a vision, man,” one boy announces to his teammates. “A deep vision.” Others struggle to find areas of discontent: one boy labours over a blank sheet of paper for 10 minutes, before eventually opting for “the stairs are too long to walk up”.

First to read his speech is Joshua, campaigning for longer lunch breaks. Joshua has written three points down, and reads them out verbatim. “It wasn’t exactly Martin Luther King, was it?” says Mr Balls, before adding politically: “It’s difficult going first.”

Next up is Yasmin. “I know all of you hate report cards,” she begins. “They act like criminal records. What would you rather have: a detention or a haunting report card?”

Hers is the outstanding speech of the lesson. But, when it comes to the class vote, they overwhelmingly opt for Adam’s campaign to change the toilet sinks. “Who wants your hands to be dirty?” he had implored his classmates. “Change the sinks!”

“Adam put comedy in it,” Joshua nods in approval. “I just read from a bit of paper.”

Mr Balls surveys his class of mini-politicians. Joshua has a point, he says: “If you want to change things, the best people to do it are the ones who are best at persuading.”

But it is not just the pupils who have learnt about persuasion this afternoon. “I’ve seen lots of lessons and lots of classes,” a drained- looking Mr Balls says, slumping in a chair as the pupils file out of the room.

“I tried to put into practice the things I’ve seen great teachers do. But it’s a challenge, holding the kids’ attention and persuading them they want to be there.”

Behind him, two boys muse over the lesson. “Out of five?” one says. “I’d give it four.”

At this, Mr Balls suddenly perks up, leaning forwards in his chair and punching high into the air: “Four out of five? Ye-esss!”

TEACH FIRST: TRY BEFORE YOU BUY IN

Launched in 2002, Teach First was inspired by the Teach For America scheme, which placed high-flying university graduates into inner-city schools.

There are currently 651 Teach First participants, working in 138 secondaries and 10 primaries across England. These schools all serve disadvantaged areas: at least 30 per cent of pupils receive free school meals, and fewer than a quarter achieve five A*-C grades at GCSE.

Three-quarters of Teach First participants work in shortage subjects such as maths, English and modern foreign languages.

For two years, they work in a single school. During the first year, they are treated as teacher-trainees, with a reduced timetable. At the end of their first year, they are awarded qualified teacher status. After two years, participants choose either to remain in their post or to move on to other careers.

Fifty-five per cent of Teach First alumni have so far chosen to stay in teaching. Two-thirds of those who leave are still involved in education, becoming either a school governor or a pupil mentor.