Teaching assistants are intended to reduce the burden on teachers
The use of teaching assistants in schools should be subject to much tighter controls, a union has said.
Unison says assistants are being used for cheap teaching across the UK.
Christine McAnea, of Unison, said the practice was "endemic" as it cost less to use support staff to cover teacher absence than to buy supply teachers.
England schools Minister Jim Knight said teaching assistants eased the burden on teachers, but should not lead classes "for more than a short period".
It costs about £150 a day to employ a supply teacher, but about £50 to pay support staff.
Rosemary Plummer, a Unison representative, said in the last few months more than 40 teaching assistants from a small area of London had told her they felt they were being asked to do more than they were qualified for.
"They're delivering maths, they're delivering literacy and marking work - that's a teacher's job... they're being used as cut-price teachers," she said.
On covering teacher absences, she added: "It's very easy to use teaching assistants on lower rates of pay - it would cost an awful lot to bring in a supply teacher."
Ms McAnea, Unison's head of education, said schools were often "putting people in front of children who may not have the right qualifications or experience to do this".
The practice had to be much more tightly regulated, she said.
In the last decade the number of teachers in England has risen by around 10% - from 399,000 in 1998 to 440,000 now.
The number of teaching assistants, however, has risen by almost 200% over the same period, from 61,000 to 177,000.
Unions are working closely with schools and the government to update the job descriptions of support staff in an effort to prevent the exploitation of assistants.
The government and teaching unions agree that teaching assistants are crucial to the smooth running of schools.
Their role was initially outlined as being to help and support the classroom teacher, and they were expected to be adept at working one-on-one with pupils or small groups.
But the guidelines on the exact role of teaching assistants are vague, and much of the role they do is at the discretion of their headteacher.
However, the government is clear that support staff should not be teaching classrooms of children unsupervised.
Mr Knight said teaching assistants are doing an incredibly good job, but should not be seen as "interchangeable with qualified teachers".
Mary Bousted, general secretary of the Association of Teachers and Lecturers union, said: "We have grave concerns about the number of teaching assistants being asked to do more than they are paid to do or feel comfortable doing.
"It is unfair on them and unfair to pupils. Heads are breaking the terms of the national agreement between unions, employers and the government if they are using teaching assistants to teach a class of pupils when a teacher is absent.
"Teaching assistants play an important role supporting teachers, but they
should not be taken advantage of by schools."
I'm working as a teaching assistant through a teaching agency. I'm frequently asked to act as a 'class room assistant' (jargon for an unqualified teacher). I have to act on my own in a classroom of children, which means essentially assuming the role of a teacher. Crucially I'm paid just £10 more, still less than half the rate of a supply teacher.
I'm a teaching assistant in a secondary school and completely understand this situation. Above my role as a TA, I'm often asked to mark work and cover lessons. It's a difficult situation - on the one hand we're not trained nor paid enough to take lessons, but we know the classes, the students and the work they're doing.
I'm a teaching assistant and work in a secondary school with year nine pupils. Because I have looked after my class of 12 children for the last two years, I often get left to do what I want with them when the teacher has things to sort out or is off sick. When they were doing their Sats I was with them during every exam and lesson afterwards, without any teacher. I never say anything about this because I'm happy that the kids are with someone they know rather than a supply teacher.
I work in a school and would say that in some weeks a teaching assistant is covering classes all day for four out of five days.
I am a teaching assistant working with students with behavioural difficulties. My colleagues and I are working with some of the most difficult students in the school but are the lowest paid teaching staff. I feel it's about time teaching assistants were given the respect and recognition they deserve for the work they do with the most difficult students.
As a retired head teacher I was very supportive of the changing role of the teaching assistant in the classroom. Good TAs know the pupils and are aware of the curriculum. They will be there to support the class teacher and are in the best position to take over should the need arise on a short term basis. This contrasts with the often poor standard of supply teachers who are put into unfamiliar situations at a moments notice and in turn do little for the pupils. TAs often have specialist skills and may actually be better able or even qualified to teach the class.
Charles Wells, Rotherham
I am a teaching assistant working in a mainstream school. I work with children who have social, emotional and behavioural problems. I plan lessons and deliver them, in one lesson I can teach as many as five subjects. I deal with many challenges during the day and often have to find a place for children who teachers are unable to cope with. I do all of this for a third of the money that many teachers get. It is frustrating and ultimately leads to me feeling undervalued and unappreciated.
I am a teaching assistant and cover supervisor. I am paid a £1 extra to take classes, unsupervised. I help plan lessons and create resources for less able students. Whilst I enjoy doing this, I do feel, that sometimes support staff are cheap labour in our school.
At last the situation has been recognised! When I started teaching in 1970, it was the sole job of the teacher to deliver the curriculum. Assistants were used to help ONLY with the non teaching tasks. I am a mother of five and was horrified when my children were taught to read by assistants who had no qualifications to do so.
Joyce Morgan, Liverpool
My son who is in his GCSE year had a teaching assistant take one of his science lessons, who knew very little of the subject! His school has employed teaching assistants to take lessons in the absence of teaching staff. How can we make these major cuts in our children's education? They are not being educated to the high standard which they should be entitled.
My wife is a teacher in a primary school where teaching assistants are employed as unqualified teachers because they are cheap. In her opinion it fails to give the children the best start in their education. These unqualified teachers are not adequately trained to deal with challenging pupils, or those with additional needs.