James Williams, deputy head of Charles Darwin School, stands quite still. His right hand is extended, palm upwards, waiting for the contraband chocolate bar to be picked up off the floor and handed over.
The giggling boy tries to give it to the teacher but drops it, and two others he has been hiding up his sleeve, causing his classmates to collapse in not-so-silent laughter. It's a scene of chaos.
Williams does not move. He ignores the giggling and taps his folder. "Put them here."
The boy picks up the chocolate bars and hands them over, head bowed.
Order is restored. The boy now stands in front of Williams, looking round at his friends. At this point, Williams pulls out a small, black book from his inside pocket. The deputy head slowly passes his gaze over the boys who have, remarkably, all decided at that moment to look down at their shoes. Then, left hand on hip, he holds up the book with his other hand and declares that it is where he records the names of people who deserve detention. Chocoholic Philip is to be the first one in it. Philip stands with his hands behind his back, closes his eyes and, before turning away, makes the half-smile, half-grimace of someone caught in the act.
Williams' role as deputy head in television series That'll Teach 'Em, which exposed today's teenagers to the schooling of the 1950s, was a break from his day job as a lecturer in education at the University of Sussex.
"A lot of what you saw in That'll Teach 'Em is poor teaching," he says. "It's teaching by fright, by fear, using snide comments and sarcasm. That sort of thing is not acceptable today. But the interesting thing is that there are ways you go about presenting yourself that are useful, very useful. A lot of things revolve around being confident and owning the classroom, standing straight with your shoulders back."
How you present yourself is just one aspect of body language, a form of communication that we all use, usually without thinking, to help get our message across.
In its most basic form, body language consists of communicating without speech. Pointing and many facial expressions are explicit and easy to understand in all cultures. But it's not always so simple: signs may be insulting in one culture and not in another, as any British person travelling abroad soon discovers as they are constantly flicked the "V" sign by Europeans to whom it simply means two. Often body language is far more subtle, sending out the message that we are bored even as we insist out loud that we have never seen a more fascinating PowerPoint presentation.
For new teachers, relying on their pre-PGCE body-language habits can undermine what they say in the classroom.
"You need to get the message across in the first few minutes that you know what you're doing," says Dr Sean Neill, senior lecturer in education at the University of Warwick. "I've done some research on the cumulative effect of the actions of teachers. Pupils took in all the information from the first action, half the information from the second thing and a third from the third action. The first four things a teacher did overwhelmed any other message.
"This is the idea behind the 'Don't smile until Christmas' mantra - whether that's correct or not. Every journey starts with a first few steps. If you fall in those first few steps, the class isn't going to take the journey with you."
Subconscious body language is something we're already expert in reading. Take the opening to the classic film Saturday Night Fever. As John Travolta struts down the street towards us we can start making assumptions about his character, even before the lyrics kick in: "You can tell by the way I use my walkI'm a woman's man, no time to talk."
We all recognise authority. Power is communicated through taking up room: walking around, extending your arms, shoulders back, head up. Feeling powerless is conveyed through the opposite: shoulders drooped, sitting scrunched up, touching your face or neck.
"When you walk into that classroom," Williams says, "it is your domain, not their domain and you have to show that in your body language.
"I say to trainees to go into the classroom where you'll be teaching before you start so you know where your desk is and their desks are, where there are things that might make you trip.
"A common mistake is the feeling that you need security, so you stand behind the desk. That puts a barrier between you and the class and 'their' space in front of the desk is larger than 'your' space behind it.
"Another one to watch once you've started teaching is that kids will sit in certain places. They get used to seeing you in one spot and they find it easy to forget about you because you're 'down there'. So move around, teach from different points, so they have to refocus."
All trainee teachers at the University of Sussex do an introductory course on voice and self-presentation that Williams, a former actor, brought in with the help of voice coach Ann Thomas. He illustrates how teachers can convey authority without saying a word.
"If a child is irritating you by tapping a pen, don't stop the lesson, just walk toward the child and take the pen out of their hand," he suggests. "It makes the point more effectively because you're giving the message 'I know it's you and I'm stopping you'."
The unspoken message demonstrates why shouting doesn't work: having to yell "Be quiet!" reveals that you're losing control.
"When you start shouting the shouts get louder and louder and you will lose," Williams says. "You have to sometimes do the opposite. If the class is loud, you go quiet. What puts the fear of God into children is very slow, very deliberate speech. What is important is not what you're saying, but how you're saying it."
Similarly, if you have a child who is shy or who is potentially frightened by authority, don't stand too rigidly: relax, sit in a comfortable manner and smile.
"I always advocate smiling," Williams says. "It's a question of finding the line between being friendly but not being a friend."
Julie Jobburn, director of studies at Warwick Junior School, has been teaching for 13 years and instinctively moves around her class. She cannot understand how teachers can bear to sit still. "I'm very conscious of positioning myself in the right place, standing towards the front of the class, and I do move around quite a lot," she says.
"I probably gesticulate a lot. I use my hands to express things and I use a lot of enthusiasm when talking. It has to be the whole thing: your voice, your facial expression. They say 'Don't smile before Christmas', but I only keep it up until day two," she says. "I feel it's important for children to feel safe and secure.
"Some teachers are naturals and do it all in a very positive way, but I do think for new teachers they can learn ways of expressing themselves, and that can be hugely beneficial."
The fascination with body language
The modern interest in body language took off in the 1960s, and one of the pioneers was Albert Mehrabian, professor emeritus of psychology at the University of California, Los Angeles. He is the originator of the widely quoted idea that communication is 7 per cent verbal and 93 per cent non-verbal; or to be more precise, 7 per cent relates to the words we use, 38 per cent to our tone of voice and 55 per cent to our facial expressions or body language.
However, Mehrabian himself regards this as a misinterpretation of his work. In an interview on Radio 4's More or Less programme in 2009, he explained how the statistic came about. In 1968, he decided to carry out an experiment on the communication of feelings. Volunteers were put in pairs and one had to say a word to the other. The words were selected to be either positive, such as "great"; neutral, such as "really"; or negative, such as "terrible". But the volunteers were also asked to communicate positive, neutral or negative feelings by altering the way they said the word. Mehrabian found that where the words and vocal elements were mismatched, it was the vocal elements that won out, the effect being roughly 5.5 times larger than that of the words themselves.
In a subsequent study, he compared the vocal elements with facial expressions and found that facial expressions were about 1.5 times more powerful than vocal elements. Combining the results resulted in the 7:38:55 formula.
"I was asking a question that was very focused on the expression of emotion," Mehrabian explained. "You can't extrapolate the findings to communication in general."
But while his study scotches any hope of explaining the Peasants' Revolt to your pupils entirely through the medium of body language, it does have important implications for communicating with others. For communication to be effective, your words, tone of voice, facial expression and posture have to match.
As Mehrabian's work did find, say "terrible" with a big smile on your face and a warm tone of voice and your pupils will only hear "great".
The University of Warwick's Neill suggests that, while it may seem false at first, would-be teachers look upon their performance in the classroom as just that - a performance.
"Teaching is a particular type of role," he says. "It has similarities with other authority roles, such as politicians, to the extent that if your way of communicating doesn't fit with what conventionally conveys authority, then it's going to be difficult for you to convey authority.
"As a teacher you are legally in charge of a group of people and you are trying to get them to learn what you want to teach them. So you have to start performing the teacher role."
Neill adds that there is some research showing that student teachers who see their lessons as a performance, and - if appropriate - come out of class saying, "Well, that was a lousy performance, I've got to give a better one in the next lesson", cope better than those who don't have the same degree of detachment.
"It is analogous to acting," he says. "If you're being a teacher you have to do things teachers do. And classes legitimately expect that. See the non-verbal part of communication as part of that performance and, while your performance may differ from other people's, it will settle down to a way of performing that is comfortable to you."
But while it may seem easier to be an actor than to spend the decades ahead pretending to have authority over towering teenage boys - or enthusiastically teaching the three times table for the 33rd time - the way our brains work means it won't be long before your teacher body language becomes second nature.
Fake it until you make it
After studying research into how the non-verbal cues you give out affect other people, Amy Cuddy, assistant professor in business administration at Harvard Business School, wondered whether they also affect you.
Or, to put it another way, can you fake it until you make it? The answer is yes.
Cuddy asked two groups of people to sit in a room on their own for two minutes. The first group was given "high power poses" to do: legs up, hands behind head. The second was asked to huddle as if they were cowed or anxious. Then they were asked to give a presentation for their dream job. The evaluators - who didn't know which group the individuals had been in - were far more likely to hire those from the power group than the others. They were seen to be more engaging and enthusiastic. When the researchers measured the testosterone levels of the volunteers, those in the power group had risen after the two-minute stretch, whereas there had been a testosterone drop in the droopy volunteers. Testosterone is the hormone associated with power and status within a group.
Cuddy's message: start acting like Wonder Woman and you'll become a superhero.
Get to know your classroom before term starts. Arrange it to suit you.
Stand at the front, or perch on your desk.
Walk around the classroom to ensure that children focus on you rather than each other - or their mobile phones.
Emphasise what you are saying with your body language: if you are enthusiastic, act enthusiastically; if you are angry, look angry.
Walk towards someone who is chatting: they will often stop as you approach.
Constantly sit or stand behind your desk.
Shout in anger. Try lowering your voice to show disapproval.
If you do need to raise your voice, learn not to raise the pitch. This is especially important for women, who have higher-pitched voices than men.
Stop the lesson unless absolutely necessary. You may be able to deal with minor misbehaviour with a stern glance without interrupting your flow.
Worry. Act like a teacher and you'll become a teacher.
Neill, S. and Caswell, C. Body Language for Competent Teachers. (Routledge, 1993).
Quintilian, Institutes of Oratory, ed. Honeycutt, L. trans. Watson, J.S. (1856). rhetoric.eserver.orgquintilian
Cuddy, A. Power Poses (PopTech talk on YouTube).
Voice Care Network: www.voicecare.org.uk.