Teaching pay scales are said to be affecting applications

The Headteachers' Association of Scotland said that unfilled posts had increased to 110, from 70.

It is estimated that just five people are applying for every vacant post, compared to up to 20 in the past.

The association's general secretary Greg Dempster said pay scaling variations were a significant factor in the shortfall.

Teaching jobs are subject to "job sizing" calculations following the McCrone agreement on education in 2002.

The system measures the "size" of teaching jobs and allocates a salary accordingly, taking into account the size of the school, its roll and the managerial responsibilities.

Mr Dempster told BBC Radio's Good Morning Scotland programme: "The results that came out of it were quite peculiar in some areas and inconsistent across the country.

Meal calculation

"You have examples where depute heads are being paid just a little bit less than the head teacher in their school and in other schools nearby, that look similar to everyone concerned, they're being paid more than the head in their schools."

He said larger schools often struggled to get applications because the differential between salaries for small and large schools was not big enough.

One of the factors in this is believed to be the use of free school meal statistics to calculate levels of deprivation.

Mr Dempster said: "If you're in a very large school with 500 or more pupils and you have a low free school meal entitlement then the salary for that headteachers post could be the same as much, much smaller schools that happen to have large free school meal entitlement."