An inquiry into how many hours heads and teachers spend covering lessons is being launched by the Assembly government.
Following a recommendation of the cross-party enterprise and learning committee, the investigation will look at how much time teachers spend covering the lessons of absent colleagues every year - and whether it is illegal under the 2003 workload agreement.
In January, the committee said teachers were covering more lessons than six years ago when the agreement was signed. But they also recognised that heads were taking a lot of the strain and stepping in to cover lessons so teachers could have statutory planning, preparation and assessment (PPA) time.
The committee's report said a government inquiry should be completed before a new clause to the workload agreement - stating that teachers should "rarely cover" lessons - is introduced this September. But Jane Hutt, the education minister, said it was unlikely the inquiry's findings would be released before Christmas.
Teachers' unions warned that the stricter rule could cost schools between pound;18,000 and pound;20,000 in supply teachers' wages.
The agreement, which applies in England and Wales, is designed to give teachers a better work-life balance and improve pupils' achievement by delegating teachers' administrative and clerical tasks, introducing PPA time and imposing limits on lesson cover. A teacher can lawfully take classes for other staff for only 38 hours a year.
But last year the committee heard evidence that heads and other senior staff were working longer hours as a result. Members were "appalled" to hear that classroom assistants were being treated like "second-class citizens", often taking on additional responsibilities for no extra pay.
They also found no evidence of improved educational standards as a result of the agreement.
The committee's report made 11 recommendations, all of which were accepted, or accepted in principle, by the education minister.
Estyn, the Welsh inspectorate, is due to publish its own report on workforce remodelling next week. It is expected to reach similar conclusions. Ms Hutt told the Senedd last week that both reports would influence government policy.
The committee's report also recommended that school managers should be appointed in larger secondaries to relieve the burden on heads, a suggestion that was welcomed by the teacher unions.
In response, Ms Hutt said she had already asked Estyn to look into the role of school bursars and how well they support senior staff.
A guide advising governing bodies how to help heads achieve a work-life balance will also be published this spring.