The inspectors' report comes as the National Association of Schoolmasters Union of Women Teachers Cymru said its members had been briefed to identify any schools that fail to comply with the national agreement.
Geraint Davies, secretary of NASUWT Cymru, said: "What needs to be reinforced, to heads in particular, is that this agreement is compulsory and contractual and has to be implemented in full.
"So far this has been more successful in certain parts of Wales than others. As a signatory to the agreement, we will endeavour to ensure it is properly implemented across every school in Wales."
Inspectors from the Office for Standards in Education in England found many teachers and senior managers were resolutely opposed to allowing assistants to teach whole classes, but did welcome an increased role for them About half of secondary schools and most special schools already give teachers the 10 per cent non-contact time promised in the deal by 2005-6.
But many primary and middle school heads, particularly those facing falling rolls, do not believe they have enough money.
"Few of the primary schools provided teachers with the proposed planning, preparation and assessment time. Most of the heads viewed their ability to do so with some scepticism and considered they would be unable to do so without funding," the report said.
Some heads increased their own teaching commitments in an effort to reduce teacher workload.
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