Teaching assistants are not getting proper training and support despite being integral to raising standards, Ofsted has warned.
Schools are failing to invest sufficient time and money in the extended workforce and focus too heavily instead on teachers, inspectors said this week.
Of 23 schools studied by the inspectorate, only six had a coherent cycle of training and professional development for all staff on how to raise pupils' achievements.
Support staff were often unsure of what they were accountable for and reviews of their performance rarely focused on personal targets or the school's overall aims.
Christine Gilbert, the chief inspector, said: "As this report has found, the wider workforce has an important role to play in improving children's achievement. However, all too often it was left to individual members of staff to identify and request professional development for themselves. It's vital that schools invest time and money in evaluating and developing all their staff, not just teachers."
The Training and Development Agency for Schools has created a career development framework for support staff to help them progress. There are also key skills on which to assess staff, which form the basis of a range of qualifications.
But only one of the schools surveyed understood the TDA's role in training the wider workforce.
Despite a lack of well-structured training, the extended workforce, including teaching assistants and family liaison officers, were praised for improving pupil achievement.
There was particular success in a number of schools in narrowing the achievement gap between different groups of pupils, inspectors said.
"They were particularly successful in engaging pupils at risk of underachievement or permanent exclusion, in developing links with the community and in re-engaging parents," the report said.
The overall effectiveness of the wider workforce, which also includes bursars and administrative staff, was outstanding in 26 per cent of the schools surveyed between September 2007 and March 2008, compared with 13 per cent in 200607.
However, there is concern that too much focus is being directed at lower-performing pupils at the expense of brighter children. Only three of the schools visited used the wider workforce to improve the achievement of higher attaining pupils.
Graham Holley, the chief executive of the TDA, said: "We know that not all schools are realising the full potential benefits of their support staff, because they are not deploying them in the best ways."
Since the Ofsted survey was finished, the TDA has launched online resources for headteachers and more than 30,000 copies of a publication on unlocking the potential of support staff have been ordered, Mr Holley added.
Concerns have previously been raised that support staff are being given too many responsibilities and used as a cheap alternative to teachers. Christina McAnea, head of education at Unison, said: "While it is great that our members are finally getting recognition for the demanding jobs they have, many will be asking, 'when are we going to get fair pay for this job?'
"Low pay is still endemic in this sector and too many schools are still shamelessly exploiting their staff."
A national negotiating body for support staff has been established with the aim of creating more standardised rules on pay and conditions. But one headteacher in Nottingham, who did not want to be named, said she thought it was wrong that experienced teaching assistants are paid more than newly qualified teachers.
In Nottingham, level 3 teaching assistants can earn up to Pounds 22,663 a year, compared with a newly qualified teacher's salary of Pounds 20,627.
"While acknowledging that teaching assistants have an important role to play in schools nowadays I am amazed at these pay scales as they do not have the same level of responsibility as a class teacher, even in the early years of a teacher's career," the head said.
"Would it be acceptable for nurses to earn more than doctors?"