Schools in Carmarthenshire have almost doubled their expenditure on agency staff in four years.
The total agency spend by primary and secondary schools was £4.9 million in 2019-2020 and £9.6 million in 2022-23.
The figures were in a report about supply staff arrangements which was discussed by the council’s education, young people and Welsh language scrutiny committee.
Julie Start, from the authority’s human resources department, said: “It’s scary to look at the figures, to be very honest.”
But she said it was more expensive for schools to employ staff directly because of things like pension costs, which Ms Start said were “spiralling”.
She and Aneirin Thomas, head of education and inclusion services, said the steep rise in agency spending was linked to time-limited grants issued by the Welsh Government to manage the impact of Covid. They both said they expected this expenditure to reduce.
Mr Thomas said: “The pattern cannot continue because the grants are not there any more.”
He also warned that the coming period was going to be “extremely challenging” financially.
Schools need supply staff to cover short-term and sometimes long-term absences, and respond to particular pressures, such as working with pupils awaiting an additional learning needs assessment.
The report said supply teachers were drawn from agencies, which paid the member of staff and charged a fee on top, or directly employed via the council.
When employed directly, it is the employer that must undertake background checks and process payments, which then entitles the staff member to a pension scheme.
There are 23 approved agencies for Carmarthenshire as part of a Wales-wide supply teachers’ framework, but schools can source staff from outside it.
Separately, a scheme being trialled in Anglesey has been set up to create a national pool of supply teachers.
The report went on to discuss the role of school cover supervisors, who supervise classrooms and ensure pupils complete pre-set work, rather than prepare lessons, mark work or teach. They are considerably cheaper to employ via an agency than a teacher.
There was evidence, according to the report, that some schools may be using teachers for work which could reasonably be done by a cover supervisor at much less cost. But it also said that schools recognised the value of cover supervisors for short-term absences, and that three-quarters of secondary schools in Carmarthenshire employed up to four of them on a permanent basis.
The report concluded that using cover supervisors for short-term teacher absences did generate a saving, but that better absence management was also needed.
Mr Thomas said teaching staff stability was one of the main ways of ensuring pupil attainment.
One of the committee members, school parent governor Ashley Butcher, said he worked as a cover supervisor and that many of them were well-qualified and very knowledgeable.
He said he was employed at a school, knew the staff and pupils, and was in his view more effective than a supply teacher with more qualifications who didn’t know the pupils.
He added that every member of staff in schools was “doing more than they should be doing.”
The council will continue to monitor the use of supply staff and update the committee on the impact of the supply teacher trial in Anglesey.