A teachers' union that backs the government's school workforce changes in England and Wales is warning they can dilute the quality of education.
Teachers now get half a day per week out of lessons for paperwork
Unqualified assistants were teaching exam classes in subjects they knew nothing about, teachers at the NASUWT annual conference, in Belfast, said.
The union is the biggest in the "social partnership" with the government - while the rival NUT is outside it.
The NUT always objected to teaching assistants being able to take classes.
Earlier this week, it announced it was taking legal action against the government over its exclusion from talks on teachers' responsibilities.
At the NASUWT conference Austin Murphy from Leeds said the problem of unqualified staff was far greater than many people realised.
"Cover supervisors will take over the whole timetable of an absent teacher, including A-level classes, for an entire term," he said.
"I do know of a school in south Leeds where a cover supervisor was asked to take on this role for maternity leave. They did GCSE and A-level classes.
"This person has no experience whatsoever in that subject.
"Clearly this is an absolute scandal. It should be known that this is happening."
The conference backed a motion warning that replacing qualified teachers with cover supervisors would "lower the quality of children's education".
A delegate from Bedfordshire, Peter Wathen, said "unscrupulous" head teachers were using cover supervisors and teaching assistants to teach GCSE maths.
"They are used as a cheap alternative. We have got to put a stop to it," he said.
A spokesman for the Department for Education and Skills said: "We have, along with social partners including NASUWT, published guidance on cover supervision setting out when it might take place and in what circumstances support staff might provide cover.
"This makes it absolutely clear that cover supervisors do not teach."