Towards the end of the 2014 school year, the mother marched into Eltham Primary School and thrust the white envelope at principal Dr Mark Thompson. No, she didn't want to sit down and chat.

Mark slowly peeled open the envelope.

A card with a reindeer in a red harness and a smiling Santa beamed back at him. "Merry Christmas," it read.

But there was more.

Principals are used to receiving complaints from parents. They might be about bullying. Changes to a school's uniform. Or in this mother's case, a belief that their child has been treated unfairly.

Fifteen months earlier she had contacted the school about enrolling her child, who had a disability. She was asked to prove that she lived in the zone. A few weeks later she wrote to say she had found a more suitable school.

Mark assumed that was the end of it. But inside the card was a handwritten message: "I hope 2015 is a year in which you can learn to open your minds and expand your pedagogical knowledge enough not to be afraid of children with a disability."

It was the legal letter tucked in there too that really got to Mark. It was also sent to the Education Department deputy secretary, the Education Minister and the Ombudsman. The mother accused Mark of discriminating against her child. That's when his wife Lynda started to notice a change in her husband.

Anyone who works in the education system knows how the pressure builds up over years. It has intensified as society changed, with schools expected to do more. Not only are schools expected to educate students, but they now have to prepare them for life in the outside world. As well as teaching students how to read and write, they need to teach them about respectful relationships, cyber security, sex education and drug education.

They need to prepare students for NAPLAN tests, provide parents with constant updates on students' progress and be available at all times, thanks to technology. Teachers and principals are dealing with students with mental health issues, drug problems and complicated family lives. Meanwhile, principals also have to run a business. They complain about being crushed by the volume of administrative tasks that were previously handled by the Education Department regions.

Today Lynda is sitting in the open-plan living area of her rented Northcote home, clutching a steaming cup of tea and talking about respect.

Something has changed, she says, since she and Mark started teaching. "The respect for teachers and principals is not there. The trust in schools is not there," she says.

"Parents feel they are entitled to tell you how they think the school should be run. And it's not from an educational point of view, it's from their own social and emotional point of view."

She places the mug on a black and white coaster. It reads "they lived happily ever after ... and then they had to go to work".

After receiving the Christmas card and legal letter, Mark became obsessed. He stayed up late at night scouring discrimination laws and drafted numerous responses. He asked Education Department staff for help. "They told Mark that he'd be fine, that he dealt with this all the time," Lynda says.

The doctor didn't think so. Mark was prescribed antidepressants and his family were asked to watch over him. They organised a dinner party at their Greensborough home to try and distract Mark from what was going on at work.

Finally, 10 days after receiving the accusatory letter, Mark had written his response. The letter was ready to post.

Not long afterwards, on December 7, 2014, an awful feeling hit Lynda as she was having a shower. She had just got home from lifting weights at the gym with Mark – a Sunday morning ritual – and had left him in the bedroom.

She grabbed a towel and raced into the bedroom, and then the kitchen, looking for Mark. But Lynda already knew her husband and soulmate wasn't there. "Something was dreadfully wrong."

The car tyres screeched as Lynda instinctively drove to the bridge. It was there that she found him.

They met at Coburg Teachers College in 1976. Mark Thompson was a joker. He made people laugh. Initially, Lynda didn't like him because he interrupted their classes with his banter. "He was always clowning around and making people laugh. I was very studious in my first year, so I found it annoying."

But he was also ambitious and already showing signs of being a leader.

Lynda and Mark connected when they realised they had the same ideology and approach to education.

"We both wanted children to succeed no matter what postcode they came from," Lynda said. "It was our job to educate all children. And we both stuck to that throughout our lives."

They started dating at the end of 1977. Lynda remembers telling her mother about Mark after their third date.

"I told her, 'I have met the guy who I'm going to marry'."

The sepia wedding photo from 1981 still hangs on the wall. Mark is grinning under his thick moustache, and Lynda is wearing a satin gown which drapes to the ground. "We were together, forever," Lynda says, " supposedly."

During the early years of their marriage, Mark became principal at Brunswick North West Primary. He was 35 at the time, and it was a lot of responsibility. He handled it well, but decided he wanted more experience in the classroom, and went back to teaching. The defining moment in his career was when he became the principal of Roxburgh Park Primary in 2002. It was a brand new school, so he could shape the culture from scratch. People loved him. Parents, teachers, students. They recently named a gym after him.

Mark was always thinking ahead. In the late '80s he began investigating how to integrate computers into the classroom. He already knew that they were going to transform education. They juggled having three children – Matt, now 32, Emma, 30, and Vanessa, 27 – with the highs and lows of working in schools.

It wasn't until Lynda became an assistant principal in 2008 that she began to fully understand what her husband was going through.

She thought about quitting a few times. Months into her new job as principal of Merna Primary School she issued a trespass notice to prevent an aggrieved parent from entering the school grounds. Eggs were pelted at her office, the side panels on her car were scratched. Placards appeared mysteriously around the school: "Stop the Corruption at Mernda Primary School."

Someone posted Lynda's number on Gumtree and said she was selling puppies. One morning when she arrived at work she was unable to open the gates. Someone had superglued the lock shut.

Julie Podbury is the president of the Australian Principals Federation. She worked as a principal for 20 years at Brighton Secondary College, and says workload is one of the biggest contributors to mental health issues in the profession. "It's a massive job," she says from her high-rise Docklands office, which overlooks boats bobbing on the Yarra River. "It has just become a huge task."

Recently, she asked a handful of principals to submit daily diary entries documenting the pressures of their jobs. This is what one principal's 11½-hour day involved:

Mark knew all about hard work. He left his job at Roxburgh Park Primary in 2008 and took up a job with the Education Department as a regional network leader. Here he oversaw the Nillumbik region as the Black Saturday bushfires tore through the green wedge, turning homes into twisted metal and ash. St Andrews, Arthurs Creek, Strathewen.

After the firestorm passed, it was his job to get everyone back to school. Within a week of the natural disaster, he had arranged temporary classrooms for grieving children who had lost mothers, fathers, siblings and friends. Every student knew someone who had died. And Mark wanted them to have a sense of normality after the trauma.

"He was supporting so many other people," Lynda says, "and he wasn't getting the support for himself."

During this time he completed a PhD at the University of Melbourne called "Primary Principal Pathways: a road less travelled". The thick black book, tucked away in Lynda's wooden bookshelf, is eerily prophetic. It calls on the Education Department to act to "reduce the stress and workload associated with the job" and to provide more support.

In May, Lynda received official confirmation that workplace stress was a major factor in her husband's death. She pursued a WorkCover dependent's claim for her husband's legacy, and won a landmark payout. Claims involving mental health issues are notoriously hard to prove.

It is not known how many principals and teachers are sick today. WorkCover claims only tell a tiny part of the story. Figures obtained by Fairfax Media show that last year, teachers and principals made 172 WorkCover claims for mental injury in 2015, up from 137 the previous year.

In the past five years mental health claims to Teachers Health Fund, a private health insurer for educators, have almost doubled. Its chief executive, Bradley Joyce, says mental health issues have led to teachers leaving the profession, put extra financial pressures on schools and hurt students' educational experience. But many principals and teachers don't reach out for help. They fear they will lose their job, or be branded weak.

An annual study by the Australian Catholic University's Phil Riley has consistently found that rates of stress, depression and burnout among Australian principals are twice those of the general population. He receives an email alert when a principal discloses in the online survey that they have thought about harming themselves in the past week, or indicate that they have a very poor quality of life. He has already received 400 of these warnings this year, and refers these principals to mental health services.

"I think workload is the main issue," he says. "They are working an average 60 hours per week. There is a lot more accountability because there is less trust in principals."

There were 1500 people at Mark's funeral. Mourners packed into the church and spilt onto the street. Teachers, academics, former students and even the Victorian Education Minister, James Merlino, who says the government is taking the issue seriously. "Being a principal is a tough job."

The government has expanded a principals support unit, which provides confidential advice and counselling, and deployed 150 extra staff to Education Department regions to assist schools. Mark has missed the weddings of two of his children, and the joys of being a grandfather. "He is just not there any more," says Lynda. "It is such a shame, such a waste."

He also missed the publication of his research into what was deterring assistant principals from taking up the top job. The study found most had no intention of applying to become principals because of the long hours, high stress levels and abuse from parents and students.

Lynda is also suffering. She returned to work after her husband's death, but her health deteriorated this year. At one point she thought she was having a heart attack. She was diagnosed with anxiety and post-traumatic stress disorder. She recently sold the home she lived in with Mark in Greensborough. Too many memories. They raised a family in that home, adding more rooms as it expanded. It's too painful to live there now.

There's a permanent tribute for Mark on the beyondblue website, and people have donated more than $23,000 to help fight depression and anxiety. They've written about what a caring, committed and passionate principal he was. One friend wrote: "Mark, you were such a wonderful person, who touched so many lives, always helping others. I wish we could have helped you."

Lynda accepts that there was nothing she could have done to prevent her husband's death. It was the result of stress that had been building up for decades, and a lack of workplace support. There are plenty of other teachers and principals working under similar conditions. Lynda wants people to talk about suicide more.

"My kids and I ask ourselves this question every day: 'What happened at that moment; why did it happen?' This is not something anyone should go through, especially over a job."

For help Lifeline 131 114; beyondblue 1300 224 636; SuicideLine 1300 651 251 (Victoria only) and MensLine 1300 789 978.

School of stress