A surge in the number of temporary teachers in NSW schools has prompted an education academic to warn future HSC results could be put at risk without stable learning environments.
The figures, released by the Department of Education, reveal the number of temporary teachers has increased by 40 per cent in five years.
More than 4500 temporary teachers have been added to the state's teaching roster since 2010.
Up to 16 per cent of the state's teaching workforce, excluding those employed to cover long-term leave, are now on temporary contracts, as 70,000 students currently undertake HSC exams across the state.
The figures, released in response to questions from the Greens at the NSW Legislative Council, show the sharpest increase occurred between 2013 and 2014, a year after the NSW government instituted its Local Schools, Local Decisions policy. The policy gave principals control over who they could hire and fire.
Professor of Education at the University of Sydney, Murray Print, said the stability of permanent employees was particularly critical in the lead up to the HSC.
"The idea of having consistency with teachers is a very important phenomenon," he said.
"What happens in schools is teachers create learning environments in classrooms, students get used to that learning environment and they get used to playing by the rules, if another teacher comes along and changes the environment then that becomes dysfunctional.
"Continuity is important as they build up strength towards the monumental [HSC] " he said.
He added that based on experience, the more vulnerable the student, the more they would be susceptible to discontinuity in their learning.
"More able students might be able to overcome these things better than others, students in more difficult situations would find that very daunting."
But the head of the school of education at UNSW, Chris Davison, said flexibility was "the new normal".
The Local Schools, Local Decisions policy gives local principals a "try before they buy model," she said, where teachers were more likely to be put on probationary contracts before getting permanent positions.
Professor Davison added the increased pressure on teachers could actually be beneficial for students, while not always being great for employees.
"What it might do is make teachers hungrier and inclined to work a bit harder," she said. "But they may feel there is also less support in terms of mentoring. They are often an invisible minority and that can be very alienating for a new teacher."
Greens MP John Kaye said the figures denied students continuity in order to bring down costs.
"It is a mean and short-sighted policy," he said. "At a time when students are having their final tests of their school achievement many will be looking back on the permanent teachers they had and the ongoing influence they had over their school careers."
A spokesman for the Department of Education said casual, temporary and permanent teachers provide schools with flexibility in meeting the learning needs of students.
