It was when head teacher John Illingworth had to stop himself from hitting a parent that he realised he needed to take some time off.
"She was a bit upset with someone else in the school and I just could not cope with the interaction."
A head teacher of a Nottingham primary school for 24 years, he went on to have a breakdown at the age of 55.
"It was brought on by an accumulation of stresses. I didn't see it coming but I suddenly found myself unable to do simple things like make decisions or relate to people.
"I realised I was ceasing to function."
He continues: "When a teacher becomes highly stressed, then their capacity to manage the classroom and relationships is often impaired."
"If you look at the case of Peter Harvey the trigger for him was the way that those children were behaving with their mobile phones.
"But if he had not been in a highly stressed state those things would not have triggered the response that he made."
According to England's Health and Safety Executive, teaching is the most stressful occupation there is.
About 80% of teachers complain about stress at work and thousands of teachers leave the profession every year - predominantly due to stress, Mr Illingworth says.
But what is it about teaching that makes it so stressful? For Mr Illingworth, it is the emotionally intensive nature of the activity.
"In a classroom of 25 to 30 pupils, all with individual needs, you have to be on top of your game - particularly if you have a challenging class."
Former teacher and educational researcher Dr Kevin Eames says the pressures of the job are very intense and draining.
"It's exciting. The adrenaline burn from the classroom is like nothing else and I've done a range of things and I keep coming back to the classroom.
"Teachers I've worked with who have come in from law, finance and journalism have commented that it is the most demanding, tiring and busy thing they have ever done," he says.
Teachers have always had to get up in front of the class and put on a performance. But things seem to be getting tougher for teachers.
According to the Teacher Support Network, which runs a well-being helpline, one in four of those who called between January and March this year described themselves as suffering from stress - compared to one in five the year before.
The helpline has thousands of calls and e-mails every year from teachers struggling to protect their wellbeing at work.
In the first quarter of 2010 the network's support services were used 49,000 times.
What is it about today's school environment that is making things more difficult for teachers?
Mr Illingworth says England's teachers are the most closely monitored in the world by Ofsted and by their own head teachers.
And there is very little down-time for teachers to re-charge and re-energise themselves."
He says: "In badly managed schools, head teachers are adding to the stress.
"If a teacher says they are feeling stressed. They often get a very negative reaction - like you had better just get on with it."
Chief executive of the Teacher Support Network, Julian Stanley, says: "Teachers should not be demonised for taking time off because of stress or other common mental health problems.
"Time away from the classroom is sometimes the best way to safeguard the well-being of individual teachers and prevent good teachers from leaving the profession altogether.
"Sickness absence decreases - and therefore standards of education for pupils improve - when schools implement meaningful policies that protect and enhance teachers' well-being."
But there is something else. Dr Eames says there has been a change in culture in recent years, which has turned pupils and students into consumers of educational services.
He adds: "If something goes wrong - it's the teacher's fault. If the exam results are not what are expected it is also the teacher's fault.
"It's this shift from pupils learning from someone who has the knowledge - to becoming consumers who are judging the providers of that knowledge - it's like a beauty contest into 'edutainment'," he adds.
Returning to the case of Mr Harvey, Mr Illingworth says cases like his where the teacher ends up striking out are fortunately very rare.
"Most teachers just crawl away broken people. This stress is destroying teachers lives and the lives of the families."
Recovered from his breakdown, Mr Illingworth has turned his experience into a positive by helping other teachers deal with their stress.
But sadly he still feels unable to cope with being in a classroom.