Workload reforms have made life harder for headteachers, had little impact on small schools and might not be sustainable.
But teachers have been "generally freed up" to deal with classroom issues by the transfer of clerical tasks to support staff, according to inspection agency Estyn.
The inspectors paint a mixed picture of the impact in Wales of the reforms, introduced since 2003's national agreement to reduce teachers' workload, highlighting positive developments as well as "many issues of concern".
"In all schools," says Estyn's report, "the work of support staff is contributing to a reduction in the workload of teachers. Greater use of support staff enables teachers to spend more time on teaching and learning issues."
And teachers found their worklife balance was better.
But less encouraging were the problems listed by Estyn. It found schools are reluctant to plan for long-term changes that have financial implications when budgets are largely unknown. Some teachers said the extra non-teaching time was negligible and most schools are not evaluating the impact on pupil performance.
"Few schools have made any link between the removal of administrative and clerical tasks from teachers and improvements in standards," said the report. "The changes implemented so far have had little or no discernible effect on pupils' standards."
In small schools there has been little evidence of workload reduction and the reforms have increased the workload of heads - reflecting findings published earlier this month by the school workload advisory panel.
Another issue is the struggle to recruit supply teachers and support staff, a problem felt most acutely by small rural schools and Welsh-medium schools. Estyn also says the Assembly government has not provided enough information about workload reduction to teachers, parents and governors.
Iwan Guy, acting director of the National Association of Head Teachers Cymru, said: "Many heads are saying the bottom line is money. The intention was excellent, but doing it is more difficult."
Dilwyn Roberts-Young, south Wales field officer for the Welsh-language union UCAC, said many Welsh-medium schools were struggling to find support staff. But they were making "fantastic efforts" to hire qualified teachers to cover non-contact time.
Heledd Hayes, education officer at the National Union of Teachers Cymru, said staff problems were being exacerbated because some schools were hiring supply teachers on a near-permanent basis.
Geraint Davies, secretary of NASUWT Cymru, said heads must ensure the agreement is properly implemented. He said PPA time has been a matter of "some celebration" for teachers and insisted there should be no shortage of staff because Wales has a surplus of new teachers.
A spokesperson` for the Assembly government said: "We have made substantial funding available - rising to pound;70 million next financial year - to help schools make the changes."