Class size has been a major issue for the past decade
The flagship Scottish Government policy of cutting class sizes will require new classrooms and extra teachers, costing hundreds of millions of pounds.
The Association of Directors of Education in Scotland conducted a survey of Scotland's local authorities.
It said councils were currently working out how to achieve class sizes of 18 pupils in primaries one to three.
The Scottish Government said its concordat with councils would deliver year-on-year reductions in class sizes.
The Association of Directors of Education in Scotland (ADES) survey, which gathered information from 22 of Scotland's 32 councils, found that an additional 2,173 teaching staff would be required, costing an extra £62m every year.
There would also need to be 900 additional classrooms, costing £360m, it said.
Murdo Maciver, from ADES, said that the strategy which lies ahead could include:
redrawing catchment areas
a phased or targeted move towards the 18 limit
school extensions and temporary accommodation
the allocation of two teachers to large classes
and a general review of school admission arrangements
In a written submission to Holyrood's education and lifelong learning and culture committee, Mr Maciver said: "It would be misleading to suggest a simple link between small classes and improved attainment.
"Smaller classes establish the conditions for better teaching, more individual attention to pupils and a better disciplined environment.
"But there is no guarantee that more effective learning and teaching will result"
The education committee has been taking evidence regarding a petition from the general secretary of the Educational Institute of Scotland, Ronnie Smith.
He has called for significant reductions in class sizes.
Class sizes has long been an issue in Scotland. In 1998 the then Scottish Office announced funding to allow progress towards a 30-pupil limit in P1 to P3.
In 2007, the then Scottish Executive implemented a further reduction, with 25 established as a new maximum in P1 classes.
Extra £40m
When the new SNP government came into power in May 2007 it soon set a new target of working towards class sizes of no more than 18 in P1 to P3.
It allocated an extra £40m to local authorities to help implement the change.
A government spokesman said: "The local government settlement specifically included a special provision to maintain teacher numbers nationally at 53,000 in the face of falling school rolls - by doing this local authorities can reduce class sizes.
"Recurrent local government budgets are increasing by 5%, 4.1% and 3.4% over the next three years.
"We expect year-on-year reductions from local government as a whole, and by each local authority over the spending review period."
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