Enough money was spent on supply teachers in the last five years to hire 2,000 full-time teachers, according to a teaching union.
Nearly £430 million was spent filling gaps in classroom provision in Scotland's 32 local authorities between 2010 and 2015, figures obtained by LBC Radio reveal.
The Educational Institute of Scotland (EIS) said more than 2,000 full-time teachers could have been hired for the sum.
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Council umbrella body Cosla said supply teachers are vital to cover absences and staff development.
EIS general secretary Larry Flanagan said: "There is now a real crisis around supply teaching.
"There were many incidents where supply teachers were unavailable and, in fact, the figure could be even higher if they had filled all the vacancies.
"We have said to Cosla over the last few years there is a crisis around supply and they have to look at a different way of approaching the issue.
"We've suggested that they should, first of all, disclose the full rate of pay for supply teachers because that was cut in 2011 as part of an agreement to save jobs.
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"That's clearly impacted on the numbers of teachers willing to do supply work.
"But also we've suggested we should look at what's called a national minimum staffing standard, which would include for every school additional cover to provide the jobs that supply teachers traditionally would do.
"That's a much better way of approaching it, because essentially the supply list is a reserve of unemployed teachers and that's not a good use of the training that the young teachers have gone through."
A Cosla spokesman said: "The bottom line is that supply teachers get paid the same as normal teachers.
"Supply teachers have a vital role to play in providing cover for absences and staff development, etc.
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"We're working with the unions, government and employers to improve the management and availability of supply teachers."
Conservative education spokeswoman Liz Smith said: "The cost of supply teaching is expensive, particularly in rural communities, and it can reflect weaknesses in workforce planning for full-time teachers.
"It is this last point which causes the greatest concern. The SNP's record on teacher numbers is very poor. These statistics suggest there is a rising cost of covering for that failure."
Labour's education spokesman Iain Gray said: "These numbers will make difficult reading for SNP Education Minister John Swinney, but it's the consequence of him slashing budgets for schools during his time as Finance Secretary.
"We know that Scotland's teachers are facing increased pressure and longer hours at work, these figures suggest a problem with workforce planning under the SNP with 4,000 fewer teachers since the SNP came to power.
"Labour would invest in our schools with a 50p top rate of tax for those earning over £150,000 a year."
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