The First Minister's commitment to slash classes to no more than 20 over the next few years will impact mainly in schools which are already overflowing, it is claimed. More teachers will have to be directed to the popular schools and new classrooms will have to be built to accommodate the expansion - many in newly built or refurbished schools. Huts may reappear.
Heads and education directors have in the past two weeks at their annual conferences increased the pressure on ministers to amend their partnership agreement pledge to cut class sizes and believe it runs counter to social inclusion aims.
Both say there are other ways to improve standards of literacy and numeracy in the early years of secondary. Schools and authorities should be left to raise attainment as they see fit, they argue.
Ken Cunningham, past president of the Headteachers' Association of Scotland, appealed to directors to speak out when he addressed their conference last week near Peebles. Michael O'Neill, education director in North Lanarkshire, also challenged Peter Peacock, Education Minister.
This week, Keir Bloomer, chief executive in Clackmannanshire, warned the national conference on rebuilding the country's schools that class size cuts would have enormous building implications.
"The question is, who's going to foot the bill?" Mr Bloomer asked. Council lawyers would be scrutinising the details. He said: "I strongly suspect that the time for this kind of centralist, interventionist policy initiative is actually past."
Lindsay Roy, education spokesman for the secondary heads' association, said that it would have advised ministers differently if it had been consulted before the announcement.
"There are considerable implications for some schools and if they are near capacity, will it mean the reintroduction of huts? Some PPP (public private partnership) schools will not be able to cope with class sizes of 20 without bringing in additional accommodation," Mr Roy said.
There were doubts about the ability to recruit the appropriate number of qualified staff and issues of equity among secondary teachers when English and maths were being singled out for cuts. Languages and social subject teachers would feel unhappy.