When Paul first went off sick, his headteacher could hardly have been more solicitous. He was told to take as much time as he needed, and the head even turned up at Paul’s house one day with flowers for his wife.

A few months later it was a different story. The school could not afford a passenger, Paul* was told. On his return to work he was given an unenviable timetable. He says it was clear that the school wanted him out, and eventually he agreed to quit.

So what changed? Paul is in no doubt that the difference in the school’s attitude can be traced to the day he was diagnosed as bipolar, a psychiatric disorder where the sufferer experiences elevated highs and chronic lows. He still recalls the interview when he told his head he was bipolar. “He said to me, `Is that a posh word for calling you a schizo?’”

The stigma surrounding mental health means teachers with problems are unlikely to get a sympathetic reaction from their schools, according to John Illingworth, a former NUT president. Mr Illingworth, who left his job as a primary headteacher after a breakdown and led a working group on the issue that reported to the union’s annual conference this year, believes the extent of the problem is more widespread than previously thought.

Department of Health figures show about one person in eight is at risk of developing a mental illness, but research carried out for the NUT working group suggests teachers may be more vulnerable than most. The union found one in three teachers will suffer from mental health problems at some point in their career. An unwillingness to disclose mental illness means the true figure could be higher still.

“We know that teachers can be quite secretive and a lot of mental illness is reported as physical illness,” says Mr Illingworth. “If people do have problems, they have a fair chance of being treated pretty badly.”

Stress is a key trigger for mental illness, and this may explain why teachers are at greater risk: teaching was found to be the second most stressful occupation, after health and social welfare professionals, in the latest Labour Force Survey.

Paul’s illness was diagnosed after he was assaulted by a pupil. An English teacher at a secondary school in Tyneside, he was on duty at the school gate at the end of the day when he was attacked by a Year 11 pupil he had previously reported for swearing - earning the boy a temporary exclusion.

In what seems to have been a revenge attack, the boy swore at him and spat in his face. The boy swung at Paul, hitting him in the face and head, before he managed to push him away.

The incident happened towards the end of the year. The boy was already on study leave and no action was taken, and the remainder of the term passed without incident. But the following term, Paul, 35, began to feel anxious, initially when he was on duty in the school yard, later in the classroom.

“I started having anxiety attacks. I felt as though I couldn’t breathe properly and I was going to pass out,” he says. He believes these episodes were a direct result of the trauma of the attack. He went to see his GP, who prescribed beta-blockers and signed him off for three months. At first his head was supportive and Paul was offered counselling, but when a return to work proved short-lived and a referral to a consultant resulted in a diagnosis of bipolar disorder, there was a sudden shift.

“His attitude towards me changed,” says Paul. “At first he thought I’d had some kind of breakdown that I would get better from, but when he found out it was more long-term, he didn’t know how to deal with it. “It was that fear of the unknown. I don’t know whether he thought I was going to come in one day and kill lots of kids or if I was going to be off work even more, but he just didn’t want to know.”

Paul went back to work the following September, but his timetable was loaded with bottom sets and pupils with behaviour problems. His room had been taken away and he was forced to take every class in a different room; he discovered another teacher had been hired to take over his workload; he was told the school could not afford to carry a “passenger”. He lasted about three months before the stress made him go off sick again. “I was made to feel not wanted there. They didn’t make any effort.”

He was summoned to a meeting with the head and the local authority at the town hall where he was told occupational health thought that he would never be able to return to work. He was given the choice of agreeing to leave with two months’ salary or staying on knowing he would be unable to go back to work and his sick pay was due to run out. He opted to quit and left in November 2008.

“They wanted me out as quickly as possible,” he says. “If I had been a pupil diagnosed with bipolar there would have been a notice on the staffroom wall about how to deal with me, but it doesn’t work the same for staff.

“I can’t believe I’m the only teacher who is going to have mental health issues at some point, but nobody knew what they were dealing with.”

A survey for the Teacher Support Network last year found that almost nine out of 10 teachers had suffered from stress in the previous two years, with two thirds claiming to have felt anxiety and 42 per cent to have been depressed. Of these, 60 per cent said issues in the workplace were responsible, with workload cited as the biggest cause, followed by the pace of change and pupil behaviour.

Julian Stanley, the network’s chief executive, says teachers find it hard to overcome the stigma of asking for help. “People feel they can’t talk about these issues in schools. They’re not going to make themselves feel vulnerable if they feel there isn’t a mechanism where they are supported,” he says.

Mental illness is often seen as a sign of weakness, he says, while school managers may not feel they have the training to deal with mental health issues. “We need to work on turning policies and procedures into practice, and a lot of that is about training and development,” he says.

Teachers can be particularly unforgiving of mental illness, adds Mr Illingworth. He believes attributes that are useful for teaching can prevent them from empathising with a struggling colleague.

“It is part of the persona of a teacher to show that you are in control: if you’re not in control in the classroom then somebody else is,” he says. “There is this `I can cope’ attitude, and you don’t get much sympathy for people who become ill.” He says teachers suffering from mental illness even get blamed for bringing it on themselves, in a way those with physical ailments never are.

He says the level of stress in teaching, resulting from workload pressure, managing behaviour problems and workplace bullying, significantly increases the chances of going on to develop illnesses such as depression and anxiety.

Stress-induced illness is a key factor in explaining the high proportion of teachers who leave the profession within three years, retire early or opt to work part-time, he adds.

He believes only a cultural change can make a difference, both in increasing understanding of mental health issues and in ensuring the emphasis shifts towards preventative work, such as involving occupational health at an early stage rather than as part of a dismissal process.

Mental health charities last year launched an pound;18 million lottery-funded campaign to try to change attitudes towards mental health problems and reduce discrimination. Time to Change features celebrities including Stephen Fry and Ruby Wax talking about their battles with depression but Emma Mamo of Mind, one of the charities involved, says there is still a lot of stigma in mental illness. “It is very much an invisible disability and there is a huge difference in the way people respond,” she says. “It creates a fear that someone is not in control and employers can be scared of that.”

The Disability Discrimination Act (DDA) of 1995, extended in 2005, gives people with mental illnesses protection in employment, but its effects seem to be limited.

A survey by the Chartered Institute of Personnel and Development found 52 per cent of organisations said they had no experience of hiring people with a history of mental health issues, and research for the Department of Work and Pensions shows employers are less likely to appoint staff with mental health problems than those with a criminal record.

“The DDA has not really worked for people with mental health problems,” says Ms Mamo. “You can have all the legal protections in the world, but it is public attitudes that need to change.”

Paul would like to return to teaching, but since leaving his school he has been unable to even look for work. Some days he feels fine, others he can’t leave the house. He has gone from earning about pound;30,000 a year to living on pound;100 a week incapacity benefit.

“Holding down a job can seem fine, but all of a sudden you find yourself hitting the buffers,” he says. He believes his symptoms were exaggerated by the assault at school, but now he knows he is likely to be bipolar for the rest of his life. He is still experimenting with medication to find a combination that will stabilise his moods as much as possible. When he does, he hopes he will be able to return to work again.

“I would love to be back working in a school,” he says. “Hopefully there will be a headteacher somewhere who will be able to see beyond the bipolar and think I can still do a good job, but realistically I know that now I have this diagnosis it is going to be very difficult to get back.”

• Name has been changed.