Increasing pressure on teachers could lead to a "crisis" in staffing, a teaching union has said.
One head teacher said experienced staff had already quit the profession over long hours, among other issues.
The National Headteachers Union Cymru said below- inflation pay awards were compounding the problem.
Education Minister Jeremy Miles told Wales Live he did not "have the resources" to increase pay beyond the 5% rise offered in July.
Louise Jones, head teacher at Maes y Morfa Primary Community School in Llanelli, Carmarthenshire, said she regularly spoke to colleagues about concerns over the retention of experienced teaching staff.
