Some classes are taught by unqualified staff and supply teachers are finding it hard to get work because of reforms in school working, a union has claimed.
The reforms were intended to give teachers more planning time
Teachers in Wales and England have since 2005 had 10% of the timetable out of the classroom to plan lessons.
NUT Cymru claims this has seen some classes taught by unqualified staff.
The Education Department said reforms allowed teachers to focus on teaching. The assembly government said assistants were not substitutes for teachers.
The changes were introduced as part of the UK Government's workload agreement which has been rolled out over several years and was intended to cut areas such as administration from teachers' workloads.
But teaching union NUT Cymru claims that since the agreement was implemented, some classes have been taught by unqualified staff.
NUT Cymru secretary David Evans said he believed it was happening in an increasing number of schools.
Mr Evans added that as a result of the changes, qualified supply teachers were also finding it difficult to get work.
It is claimed some classes are being taught by support staff
He said: "Some secondary schools use teacher agencies rather than their local authorities when they require cover.
"We have members, in their later 40s and early 50s, who left teaching to bring up a family or to care for an elderly parent. They later wish to return to the profession via supply teaching.
"Until recently, they would have been taken on fairly regularly by three or four comprehensive or primary schools.
"Increasingly, however, we find that they have had only a handful of supply days and those, possibly, at only one school.
"Many supply teachers feel strongly that the new pay and staffing structures together with the use of agency teachers have left them with no in-service training, no pension provision and, to all extent and purposes, with no job."
The UK Department for Education and Skills said the school reforms were benefiting teachers.
A spokesman said: "Our workforce reforms are turning the tide on teacher workload and ensuring that teachers can focus on what they do best - teach.
"They remove a whole range of administrative tasks from teachers, give them 10% of the timetable to plan, prepare and assess work outside of the classroom and limit the amount of time they can be asked to cover for absent colleagues, all helping to benefit pupils.
"We also have more teachers than at anytime since 1980 as well as record numbers of support staff.
"We continue to work with an expert panel of heads to slash the bureaucratic burden on schools and ensure that any policy changes consider the workload impact on teachers."
A Welsh Assembly Government spokesperson explained that the workload agreement was an issue of pay and conditions and was not devolved to the assembly.
But the spokesperson added: "Teachers and support staff are not interchangeable.
"Responsibility for pupils' overall learning outcomes will always rest with qualified teachers.
"Support staff will only work with pupils if they are suitably qualified and experienced and under the direction and supervision of a qualified teacher.
"It will always be the responsibility of schools to ensure that the level and mix of staff employed are sufficient to meet pupils' needs.
"Schools must not use teaching assistants as substitutes for teachers as this would be in breach of the national workload agreement."