The shortage of teachers in primary and secondary sectors in Scotland, though not yet at crisis point, is beginning to hit hard, particularly with regard to supply.
The problems could lead to the end of daytime continuous professional development, including traditional in-service days, in some local authorities and teachers being asked to undertake training after school hours, even at weekends, instead.
The City of Edinburgh's employment manager, Tom McMillan, says the biggest problems his authority has are in primary schools, because secondaries are better able to cover for absences.
"Primaries clubbing together to pool supply resources might help, and this is an idea we're piloting in Wester Hailes. But the bottom line is there are just not enough teachers and all strategies we attempt will be affected by this," he says.
In the secondary sector, Mr McMillan expects to have serious supply problems next session in computing, maths, religious and moral education and music.
"Daytime staff development is soaking up supply, so that there is none left to cover for illness. I think the answer might be to have all staff development out of school hours and at weekends on a paid basis.
"There's an immediate problem that needs a response."
Bill Milligan, president of the Association of Head Teachers Scotland (AHTS), which represents primary heads, says that in his own local authority, South Ayrshire, the spectre of non-daytime staff development has been raised and he describes it as "a serious runner".
"How staff who work their socks off day to day will react to weekend in-service I don't know. And there's also the question of how it would impact on their stamina and creativity as teachers.
"This is the 21st century, not Victorian times. It might work on a voluntary basis, but if it was imposed I can see there being problems."
At his own school, Dalmilling Primary in Ayr, Mr Milligan has first-hand experience of how desperate the supply teacher shortage can be. "On one recent occasion we had to make 71 phone calls to get temporary cover for a P7 class - and we were unsuccessful. That has been the worst; but on several occasions we have had to make 50 or 60 calls.
"When a teacher goes out on staff development we always make sure we can cover within the school. We're forced to because of the lack of supply, but it impacts on the amount of daytime CPD staff can be released for," he says.
"We do this by drawing senior staff away from their duties in our special education support for learning facility, and we can only do that because they don't have register classes."
The supply shortage is particularly difficult for Dalmilling Primary because its catchment area is in the top 14 per cent of multi-deprivation postcodes in Scotland, but as president of the AHTS, Mr Milligan knows it is a pretty bleak picture across the country.
"I can think of four occasions when supply teachers have refused to come to our school because of the area. They hear the name of the school and won't come! That angers me and it's not fair.
"But over the whole of Scotland it's inordinately difficult to get staff cover.
"In Fife, some of our members were teaching classes of 60 rather than send the class home. We invited them to stop because of health and safety implications.
"The director was concerned, as was the Scottish Executive, and the directorate sent in primary advisers to take some of these classes," he says.
The AHTS knows of long-term shortages in primaries where a teacher can be off for three or four months and "either no supply or a series of supply teachers" come in. "Either wat disrupts the children's education and it's a serious situation for the class concerned."
In the immediate future Mr Milligan sees a continuing shortage throughout the country for three reasons. First, there is not the pool of staff. Second, he believes the Executive's promise of 4,000 new teachers, in response to the McCrone report, fails to take into account replacements for those retiring. And third, the public perception of teaching and work conditions remains highly negative.
"Successive governments have been beating teachers for years and the public perception of the profession remains negative. The Thatcher years were horrendous for teachers and the Chris Woodhead years were worse and in the past inspections have been less than fair, given the economic conditions. "All these things have a cumulative effect and put people off teaching, despite its rewards and satisfactions."
Cover for special educational needs teachers is also proving very difficult, says Mr Milligan, because there are not many supply teachers with SEN qualifications.
"In these circumstances you have to make do with ordinary class teachers. How special schools cope I can't imagine, but the AHTS has anecdotal evidence of serious problems."
The shortage is also going to have a serious impact on the Scottish Executive's proposed reduction of class contact hours to 22 for primary and special needs staff.
"That's a reduction of 2.5 hours per class per week. How is that going to be achieved without the adequate cover?" wonders Mr Milligan.
There are more questions than answers about McCrone" - that's what worries Andy Gray, depute head at St Augustine's High School in Edinburgh, who has been working with the city's education department on a survey of how teacher shortage is affecting secondary schools.
"Continuous professional development is obviously very important but I don't think the McCrone report fully addresses this. Anything that stops the interruption to learning and keeps the planned interruption of CPD to a minimum, has to be the way forward in the very challenging situation which we face over supply cover.
"There has to be a balance but I'm in favour of CPD after school. I think it will become inevitable," says Mr Gray.
Although the outcomes of the survey have still to be discussed by a pilot group, it seems likely that an on-line register of subject supply teachers will be set up. Edinburgh already has one for the primary sector. The problem will be keeping the register updated.
"If a headteacher dips into the pool but forgets to tick the supply off, it leads to confusion and wasted phone calls, as sometimes happens with the primary system," says employment manager Tom McMillan.
"But the bottom line here is that an on-line facility, no matter how efficient, does not itself create one new teacher."
However, it will mean a move away from schools having their own private network of cover which, says Mr Gray, has become a coping strategy we have to move away from.
"If you have a specialist drama teacher, he or she should be using that specialism in a number of schools rather than, for example, taking some drama classes and babysitting other English classes in one school.
"Raising attainment demands a sophisticated system to get the specialists where they're most needed. The fact is, pupils should have a specialist teacher if we are to provide a high quality service. It should be as with full-time posts: the best qualified person should be employed."
The survey suggests that specialist shortages in Edinburgh are worst in computing, maths, religious and moral education, home economics and craft and design, he says.
St Augustine's has its own coping strategies in place. "We've made some of our cover pool part of our normal staffing. We have a budget for supply cover and can use that to extend part-time