Weak leadership in half of schools threatens to undermine the deal to cut teachers' workload, inspectors said this week.
Management failures damage children's education and mean that many schools are ill-prepared to reduce the administrative burden on teachers and provide them with the guaranteed time for planning, preparation, and marking promised by ministers.
Poor heads fail to monitor their staff's workload, risking them becoming overburdened. This results in a reduction in the quality of support to pupils, according to an Office for Standards in Education report.
Other heads create a culture where staff feel unable to express their views without prejudicing their careers, it warned.
One teacher said she had almost left the profession after teaching at a school where asking for help was seen by senior staff as a sign of weakness.
The survey of 150 schools carried out by inspectors between autumn 2002 and spring 2003, found that two-fifths of schools had leadership weaknesses which were likely to cause difficulties, despite "generally good" management.
In addition, one in 10 schools had management weaknesses and failed to make effective use of the skills and strengths of staff, including teaching assistants.
Heads' reluctance to use assistants in a teaching capacity left them feeling "undervalued and resentful", inspectors said.
They attacked the "insensitive" treatment of support staff, which in some schools included their exclusion from the staffroom during break times.
Deployment of support staff was less effective in secondary than primary schools with teachers often continuing to carry out routine jobs such as collecting money for school trips and putting up displays - two of the more than 20 tasks they no longer have to do.
Teachers in primary and secondary schools were reluctant to delegate tasks because they saw them as part of their job.
The report said: "The potential of teaching assistants was not realised fully, teachers spent time on unnecessary tasks, and the quality of education was less good than it might have been."
By contrast, effective school leaders had limited the amount of time spent in meetings, ensured that professional development was part of staff's day-to-day life and provided training for teachers on how best to use support staff.
The report was published amid growing complaints from school leaders that they do not have the resources to implement the agreement effectively. The National Association of Head Teachers will decide next month whether to withdraw its support for the deal because of concerns over funding.
Ofsted suggests that school buildings may limit the benefits of planning and preparation time as few primary schools had adequate private study areas for teachers.
In some areas, heads are being forced to appoint inappropriate teachers because of staff shortages, it said. Heads were often reluctant to use pay flexibility to reward the best staff for fear that could be divisive.
David Hart, general secretary of the NAHT, said: "This report vindicates everything the we have been saying about the difficulties a large number of schools face. There may be some schools with weaknesses in leadership but the vast majority of difficulties are due to a lack of confidence that the money will be there to implement the agreement."
Leadership and Management: managing the workforce is available from www.ofsted.gov.uk
Steps to effective management
* Environment: invest in school buildings and tackling bad behaviour. Good managers are aware of the need to promote worklife balance.
* Staff: use performance management and training to develop staff. Policies should ensure the right people are appointed to the right jobs.
* Change: explain the benefits of change and take time to introduce new ways of working. Review the effect of managers' decisions on staff.
* Culture: create an open climate where staff work together towards a clear set of targets.