Cover for unplanned absences is having serious repercussions for headteachers' workload and their plans for developing the school.
In a conference report, Gordon Smith, AHTS vice-president and head of Jordanhill primary in Glasgow, complained that a head in North Lanarkshire had had to do 30 consecutive days of supply cover because of a shortage of supply teachers.
Others were class-committed for "unacceptable periods". The amount of cover heads undertook depended on size of school, time of year and number of staff with no class commitment.
Mr Smith says that in schools with management time, the problem is "delegated" to other promoted staff without a teaching commitment.
Development initiatives such as team teaching, curricular reinforcement and working groups are frequently cancelled.
"Development planning is very difficult to manage when key staff have to cover unexpectedly for colleagues. Other provision of in-service is cancelled as teachers cannot be released due to lack of supply teachers available for cover."
The conference heard repeated concerns about heads being forced to provide extra cover as class teachers' hours are gradually reduced. Jim Smith, Wellhouse primary, Glasgow, said the issue was a "huge iceberg" that would have enormous repercussions.
Judith Gillespie, development manager at the Scottish Parent Teacher Council, warned heads that parents would be angered when they discovered the cuts in teaching time with the individual class teacher.
* Self-evaluation is well established in two out of three primaries, Graham Donaldson, senior chief inspector, told the conference. The best primary heads were every bit as good as managers anywhere in the public or private sectors but headteachers were still failing to monitor the work of classroom teachers.
"In our inspections we find too often that the level of close monitoring is not high and poor or unproductive practice remains in place as a result," Mr Donaldson said. But leadership was improving. Between 1992 and 1995, it was rated as good or better in 75 per cent of schools and between 1998 and 2001 this had risen to 85 per cent.
"We have to address the 15 per cent of schools where leadership requires significant improvement," Mr Donaldson said.