Scottish councils are substantially cutting back on their use of supply teachers, raising concerns about the pressures on existing staff to meet the training demands of Curriculum for Excellence and exam reforms.
Unions and opposition politicians have blamed the drop on spending cuts and a worrying shortage of supply staff.
Among the 20 councils that provided data, the number of supply teaching hours purchased fell by a total of 333,682 between 2010-11 and 2012-13, or 15 per cent. And the 14 councils able to offer figures for the school year just ended had experienced a 16 per cent drop since last year, amounting to 171,838 fewer hours in total.
The decrease was particularly dramatic in some areas such as Dundee, where supply hours dropped by about a third.
The figures, obtained by the Scottish Liberal Democrat education spokesman Liam McArthur through a Freedom of Information Act request, were revealed after an Audit Scotland report showed that council spending on education as a whole fell by 5 per cent between 2010-11 and 2012-13, largely owing to fewer staff being employed.
In December, a TESS survey revealed a shortage of supply teachers in 23 of the 25 local authorities that responded. One said the situation had reached "crisis point". Unions said the new figures offered further evidence that some areas did not have enough supply teachers.
Some schools and local authorities have been taking steps to reduce their need for supply cover by, for example, holding training courses outside the school day. Mr McArthur said that although he welcomed innovative approaches such as these, he remained concerned that the supply crisis in many regions had not been resolved.
He also said that the implementation of Curriculum for Excellence, combined with falling teacher numbers, had placed "enormous pressure on all teaching staff".
He added: "Instead of allowing for well-supported CPD, this cut in supply hours suggests that teachers are being put under tremendous strain, with pupils at a potential disadvantage."
Teachers had to know supply cover would be available "if and when they needed it", Mr McArthur continued.
"However, the figures we have revealed, along with evidence from teachers across the country, suggest that they cannot have this confidence. Ministers must provide details as to how they will ensure that there are enough teachers available in all areas, for all age groups and all subjects."
A spokeswoman for the EIS teaching union told TESS the union was "concerned that such a drop confirms local reports that supply teachers are thin on the ground and that in some areas there is a real and significant problem".
The spokeswoman explained that the Scottish Negotiating Committee for Teachers (SNCT) had set up a working group to identify "practical answers to common issues with the availability of supply teachers".
"The payment of supply teachers and other provisions in the SNCT handbook will be looked at and there will also be a consideration of the interaction between supply and teacher workforce planning," she said. "The EIS will play its part on the teachers' side of this group and will pursue and push for solutions."
Ken Cunningham, general secretary of School Leaders Scotland, said the Scottish education system was now "very heavily dependent" on supply to cope with the development and CPD needs of staff, particularly with the advent of the new qualifications. He added that in remote areas some subjects would face difficulty, particularly in cases of long-term absence.
He said: "Better consistency in approach to appointing and dealing with supply across authorities would also help."
A spokesman for the Scottish government said: "Staffing levels in schools have been broadly safeguarded over the past three years as a result of the Scottish government's shared commitment with local authorities to maintain teacher numbers in line with pupil numbers. In addition, the proportion of probationers securing employment immediately after their induction year has increased year-on-year since 2009.
"We recognise that schools and local authorities have faced challenges securing supply cover and that's why the most recent SNCT pay agreement made changes to the pay rates for supply teachers."