High school teachers are leaving the profession after being placed in subjects they aren't qualified to teach. More than a quarter of year 7 to year 10 teachers and 15 per cent of year 11 to 12 teachers in Australian schools are teaching a subject they have not studied above first year at university and for which they have not received training in teaching methodology, according to the latest Australian Council for Educational Research (ACER) figures. The implications of this on retention rates and the mental health of teachers are widely overlooked, says Dr Du Plessis, a research fellow at the Australian Catholic University who is set to release a book in June on teacher burnout. "You feel guilty because you don't have an in-depth knowledge of the subject or the skills or knowledge to guide students, especially if they're in year 11 and 12," she said. "If you do this over a long period of time, you get exhausted and feel you're not achieving what you want to achieve. It can lead to burnout."

In some of the worst cases Dr Du Plessis has come across, teachers have been put on stress medication and needed counselling to cope with the burden of roles for which they don't have the proper training. About 37 per cent of teachers in their first or second year of teaching are in out-of-field roles compared to 25 per cent of those with more than five years' experience, meaning younger teachers tend to be disproportionately affected. In one such case that Dr Du Plessis looked at, a science teacher in her second year had been given physical education classes, but wasn't comfortable teaching in an open space. "She said if things didn't change, she would leave the profession," Dr Du Plessis said. "I think that's severe, to lose a science teacher because she's in an out-of-field area." President of the NSW Secondary Principals' Council Chris Presland said burnout is common among teachers placed out of their field.

"They're on this constant treadmill of just trying to keep ahead of the kids," he said. "They're frustrated and they'll look for another career. That happens a lot." Australian schools are experiencing the greatest shortage of qualified teachers in technical areas. In years 7 to 10 about 41 per cent of teachers IT classes, 23 per cent of teachers in physics classes, and 21 per cent in maths classes do not specialise in these areas, according to a 2016 ACER report. "It's hard to solve the real root cause of the problem, which is that for people with maths, science and IT credentials, a teaching salary is nothing in comparison to how much they can get in industry," Mr Presland said. "The first implication [of this shortage] is that if a school offers a technical subject, they're likely not to have qualified teachers."

"The second is that schools may not offer the subject at all." Students in junior classes with an unqualified teacher are less likely to pursue the subject in later years, he said. Out-of-field teaching is far more prevalent in schools with a low socioeconomic status; 31 per cent of year 7 to 10 teachers in these schools are in out-of-field areas, compared to 22 per cent of teachers in well-off schools. In remote areas, 41 per cent of teachers are in out-of-field classes, compared to 24 per cent of teachers in metropolitan schools.