An Ofsted survey of 29 schools which had strong continuing professional development found that few schools properly evaluated its impact on pupils.
It said schools should hire consultants to advise on subjects in which there is a lack of knowledge and should make greater use of coaching and mentoring for staff.
The survey was supported by inspection evidence of subjects in 130 schools.
It showed CPD was inadequate in at least one subject in a third of primary schools and that primaries' focus on literacy and numeracy led to a neglect of training in other subjects.
Ofsted said schools too often relied on teachers' own assessment of their training needs.
"Managers observed lessons, but their comments rarely contributed to the process for identifying teachers' professional development needs," it said.
"Teaching assistants were seldom formally observed working with children in order to identify how they could improve their practice."
Some managers in secondaries were opposed in principle to using support staff in the classroom, while one head described the idea as a "total anathema".
Teachers' professional development was weak in schools which lacked expertise in particular subjects.
The report said: "In one school, the sole art teacher was unaware of gaps in her knowledge of contemporary art. Furthermore, there was no mechanism by which senior managers could pick up this limitation. Put simply, they did not know what they did not know."
Inspectors called on the Training and Development Agency to encourage more subject specific training in primary schools and to help schools to identify the needs of individual members of staff and said schools should put CPD at the heart of their efforts to raise standards and do more to assess its value for money.
Jane Joyner, Ofsted curriculum and dissemination manager, said: "Continuing professional development is most effective in schools which understand and realise its potential for raising standards."
"The logical chain: continuing professional development in effective schools" is available at: www.ofsted.gov.uk