Marianne Knightley* will always be haunted by the spectre of what might have been. Occasionally, she recalls that moment in the playground when, tears coursing down her face, she was unable to move, unable to think. She remembers the pupils looking on in wide-eyed silence.

She thinks about the single phrase, repeated again and again: “I can’t do this; I can’t do this.” But, mostly, she thinks one single, horrifying thought: I could have turned violent.

The extreme emotions that a tough day in the classroom can provoke are not unfamiliar to most teachers. But while most are able to separate the personal from the professional, take a deep breath and plough on with the lesson, not all can. Earlier this year, Nottinghamshire science teacher Peter Harvey was cleared of attempted murder after hitting a 14-year-old boy around the head with a dumbbell, screaming “die, die, die”. The boy had sworn at him in class.

“I imploded, rather than exploded,” says Mrs Knightley, a primary teacher from Nottingham. “I’d never cried in front of the children before. But once I’d started, I couldn’t stop for hours and hours: I had no control over what I was doing.

“My collapse went inward. But it could have so easily gone the other way. It could have been an explosion. When I heard what Peter Harvey had done, I thought, `There but for the grace of God go I’.”

Mr Harvey is an extreme case, but he is not unusual: most teachers will have stories of colleagues who lost control in the classroom less violently, but no less suddenly or dramatically. “When I taught, we had one particular teacher the kids nicknamed Volcano,” says Jon Berry, senior lecturer in education at Hertfordshire University. “Their job was to get him to erupt. There’s a volcano waiting to explode in every school.”

But far from recognising the potential for disaster, he says too often both schools and teachers bury their heads in the sand until it is too late. “Complacency is the enemy of any teacher,” says Mr Berry. On one occasion during his teaching career, he became aware of the sound of a teacher unleashing his fury on pupils - to little effect - in the next room. He approached the teacher afterwards to see if he needed help.

“That sounded pretty horrible,” Mr Berry ventured, chattily. “No it didn’t,” the teacher responded, shutting down the conversation immediately.

“These people blame the kids,” Mr Berry says. “They say: `What would you expect from 9Z? They’re a lot of little bastards’.”

Mrs Knightley, however, is clear that what happened to her was not her pupils’ fault. “A child was the catalyst,” she says. “But the stresses of what I was being asked to do, the lack of support: all these things played a part.”

An experienced Year 2 teacher, Mrs Knightley had worked hard to help her school through a difficult period: two flooding incidents, a merger with another school, new buildings, a new head and slipping results. “I was brought up to do any job I was given to the best of my ability,” she says. “But I had so many jobs to do, I wasn’t doing any of them properly.”

She found herself doing something she had never done before: losing her temper with pupils. One child in particular began to trouble her. He had extreme special needs and regularly refused to follow any instructions. Instead, he would crawl around on the floor, deliberately tripping her up. The other children merely laughed; one boy with autism began copying him.

By autumn half-term, she was struggling to get up in the mornings. Her life felt like a maze of dead ends. “By October, I felt like it was July,” she says. “I was permanently tired, permanently stressed.”

Ed Stewart* was a newly qualified teacher and, like most in his position, anxious to make a good impression. He was brash, confident, full of self- congratulatory anecdotes. He had just started his first job, as an English teacher at an inner-city comprehensive and, as far as colleagues were aware, everything was going smoothly.

Of course, his Year 9 pupils had begun testing his limits. But that was just the way with Year 9. He could cope. The stories he told about his work tended to be humorous, upbeat, the stories of a teacher in control of his pupils. By the time the cracks began to show, they were earth- splitting fault lines.

As autumn term progressed, Mr Stewart found himself struggling more. His Year 9 class was still playing up - if anything, the pupils were getting worse. They refused to follow basic instructions, to behave when he told them to. By November, as winter darkness wrapped itself around the school, the classroom began to feel increasingly claustrophobic. His pupils were simply not co-operating; teaching became impossible. He knew he was losing what little classroom control he still had.

“It’s that feeling in the pit of the stomach,” says Sion Humphries, Mr Stewart’s deputy head. “You feel yourself getting hotter and hotter. It’s a form of anxiety attack. It’s not a nice feeling at all.”

It is, Mr Berry argues, a feeling many teachers will recognise. “Most teachers, on an everyday basis, will have some point when they are on the knife edge,” he says. “Their overall and complete control is uncertain, albeit temporarily, and it feels frightening. In terms of your reputation, both with children and colleagues, the way you conduct yourself is absolutely central.”

Most teachers know this; most manage to fall on the right side of the knife edge. But those who are vulnerable - who lack experience, or who are having personal problems - can struggle to claw back control.

Peter Vaizey* was a relatively experienced teacher at a Welsh comprehensive. His own child was gravely ill and he was desperately worried about the situation. His pupils sensed his distraction, his waning confidence in his ability to cope with life, to resolve the challenges it placed before him. And they took advantage. Eventually, Mr Vaizey was pushed to his limit, and ran from the classroom in tears.

“When lions hunt wildebeest, they can pick out the nervous one, the one that lags behind, and that’s the one they go for,” says Mr Humphries. “Pupils are similar: they see weakness and adopt lion-like behaviour. If you’re a wildebeest, your only option is to be early lunch.”

This was something Mrs Knightley knew. By April, her troublesome pupil was acting up almost every day. He would spend entire lessons kicking the classroom door. At other times, he refused to leave the classroom at breaktime; his teacher, therefore, had no break either.

Then, during an outdoor lesson, the child threw himself onto the playground floor and refused to stand up again. Mrs Knightley tried reasoning with him quietly but got nowhere. Then she tried to persuade him to move towards the classroom by invading his personal space. Still nothing. Eventually, she sent her teaching assistant for help. If she was going to have to move the child forcibly, she needed a witness.

No one came. “The message was: you’re on your own here,” she says. “It felt like nobody was bothered. I sat down on a picnic bench. I sat there; the boy was lying there. I thought, `This is quite a pleasant way to spend a working morning.’ Then I thought, `I’ve spent years training to do this.’ And I just started crying.”

For a while she just sat there, tears pouring down her cheeks, repeating: “I can’t do this; I can’t do this” over and over. Then she wandered into school, dazed and directionless. She had no sense of where she should be, what she should be doing. She felt helpless. She knows that she was taken into the staffroom. Somebody hugged her; a cup of coffee was placed in her hands.

“Are you all right?” they asked. “I can’t do this,” she repeated. “I can’t do this any more.” Eventually, the head suggested she go home. “It’ll be fine,” Mrs Knightley thought, as she drove. “I’ll feel better in the morning.” It was another 11 months before she returned to the classroom.

For Ed Stewart, also, the ultimate trigger was nothing particularly remarkable: some low-level disruption, refusal to comply with instructions, an unwillingness to respond enthusiastically to a lesson he had carefully prepared. But, one November afternoon, he too just snapped.

First he yelled at the pupils, telling them to shut up and listen. Then he started swearing at them. “He generally gave them a bit of a character reference,” says Mr Humphries. “It was the equivalent of lashing out when you’re cornered.”

The teacher in the adjoining classroom heard the noise and sent a pupil to fetch support. By the time Mr Humphries arrived, it was to see Mr Stewart run from the classroom in tears.

When Mr Vaizey, in his Welsh comprehensive, also fled from the classroom in tears, he knew that he was no longer able to cope with his problems alone. He went to see his deputy head and explained what had happened. The deputy was Brian Lightman, now general secretary of the Association of School and College Leaders. “These things can be rescued,” he says now. “The children knew they had overstepped the mark. They knew they had to put things right. What needs to be brokered is a resolution.”

He went into Mr Vaizey’s classroom and, after telling pupils off for their behaviour, explained the background to the situation. “The children were very apologetic, very contrite about it all,” he says. “They promised they would never behave like that again.”

Ed Stewart’s class, too, was nonplussed by his outburst: there was no sense of elation, no feeling of a battle well won. Instead, the raw humanity of their teacher’s response caught the pupils unawares.

A senior teacher found Mr Stewart and sent him home for the remainder of the day, making clear that this was not a punitive measure. Meanwhile, Mr Humphries stayed with the class for the rest of the lesson. “I thought I may as well strike while the iron’s hot,” he says. “The first thing in a situation like this is to lay down ground rules. `We’re not here to discuss Mr X personally. We’re going to talk in broad terms about what’s happened.

”`A teacher was distressed, as you saw. How do you feel now? Do you feel that you were responsible?’ It has to be seen as an opportunity for learning and development. Then they feel part of the solution as well as the problem.”

“Most teachers will claw it back,” Mr Lightman agrees. “Most teachers will be able to take whatever steps are necessary to stop that kind of thing from happening again. But you need a structured way of dealing with it.”

This was what Marianne Knightley did not have. Back at home, she found herself experiencing panic attacks every time she tried to make a decision. It took her months merely to be able to choose whether to stand up or sit down without crying. But she felt her head was consistently unsympathetic. Mrs Knightley was stripped of her management responsibilities and issued with an official warning for failing to produce a return-to-work plan promptly.

Eventually, she agreed to return part-time. And so, she revisited the scene of her meltdown. As she sat in the head’s office, children began banging on the window: “Mrs Knightley’s back! Mrs Knightley’s back!” The head drew the curtains. Staff were less vociferous. One colleague came and hugged her: “It’s so nice to have you back.” No one else acknowledged her presence; she had to speak to most of them first. “It was as if I was invisible,” she says. “I felt like I was a horrible person. I felt like a leper. I wanted to walk around with a T-shirt saying: `It’s not catching, y’know’.”

Ed Stewart, too, was faced with the difficult task of returning to the battle-scene. “The longer it takes before you’re back in the saddle, the more difficult it becomes,” Mr Humphries advised him. “It grows and grows in your mind. Better to confront it fairly quickly.”

And so, the day after his outburst, he walked into school with sheepish trepidation. But the previous day’s lesson had been effective: as he walked down the corridor, several members of Year 9 approached him to apologise.

Together, he and Mr Humphries discussed his return to the classroom. They talked about what he might have done differently, how he might have prevented the situation from escalating. For the brash, self-confident young teacher, this was not an easy process: he was forced to step back and admit error.

Similarly, he needed to learn to ask for help. “There can be a temptation to think, `I cocked it up last time; I’ve got to be world-class this time,’” says Mr Humphries. “But, actually, you don’t. Don’t go for something risky or out-of-the-ordinary. Go for something safe. A satisfactory lesson is fine under the circumstances.

“Don’t try to be something you’re not. That’s a recipe for disaster. Young people are superb amateur psychologists: they can spot a phoney a mile off. Just be yourself.”

Mr Stewart walked back into his Year 9 classroom the same outgoing, brash teacher his pupils had previously known. Though his wings had been clipped, his pupils could not tell. But, tempting though it was to ignore what had happened, he knew he could not - he had to make it clear that there were no hard feelings. Unexpectedly, his natural laddishness helped. He bantered his way through the conversation, making self-deprecatory jokes. The class responded well; Mr Stewart went on to complete his induction year successfully.

Peter Vaizey, too, managed to prevent his career from being torn to shreds, wildebeest-like, by ravening pupils. It was agreed that his head of department would teach the first part of the lesson; Mr Vaizey would come in later, once a sense of calm had been established.

This supported lesson was negotiated beforehand: Mr Lightman was aware that some teachers may perceive this option as a sign of weakness. Alternative approaches mooted included a visit from a senior teacher midway through the lesson and a visible presence in the corridor. Mr Vaizey was reminded that asking for help occasionally is neither shameful nor unusual.

“Teaching is a bit like sport: you need a bit of nervousness to keep yourself sharp, to keep your wits about you,” says Mr Berry, of Hertfordshire University. “But it’s critical to seek support, help and advice. It’s not weakness to seek support.”

Indeed, however great the fear of appearing weak in front of colleagues may be, it is nothing compared with the terror of sudden, irretrievable loss of control.

“In that moment, you could do anything,” says Mrs Knightley. “It’s horrible. If that child had been in my face, I don’t know what I would have done. If I had started to move him, to push him, I don’t know when I would have stopped. There but for the grace of God.”

She has now been back at school for three years. But she has struggled to put the incident behind her. “It’s incredibly difficult,” she says. “If I’m honest, it’s difficult every single morning. But you manage. There’s something about the kids that keeps you going. But it’s never gone away, and it probably never will.”

* Names have been changed.

• Original headline: Cover Story - `There but for the grace of God .’