The National Union of Teachers and Secondary Heads Association have been opposed to teaching assistants taking classes since the idea was first floated by Education Secretary Estelle Morris last November.
But now some of the 32 schools in a pound;4 million government project to try out ways of cutting teachers' hours have said that they will experiment with assistants covering for teachers. Teachers currently work on average 50 hours a week.
Bob Pearson is head of Campion Catholic high, Liverpool, one of the "pathfinder" schools trialling ways to cut workload, and has won pound;300,000 to do so. He intends to guarantee one day a week of non-contact time for teachers. In order to do this, he said, teaching assistants would have to supervise classes.
He said: "We are looking at how we can train and develop people who are not teachers to help with supervising classes. It is a revolutionary step. At the moment another teacher will cover until we can get a supply teacher in, but we have some very experienced people who could do supervision if we set up resources.
"We want to increase the amount of planning time, but more importantly we want to guarantee planning time. Teachers have always had non-contact time but often this is taken from them because they have to cover for absent colleagues."
He added that teaching assistants who know the children may find it easier to manage behaviour than supply teachers who do not.
A survey of its members by the National Union of Teachers found that nine out of 10 were opposed to assistants taking charge.
But John Bangs, NUT head of education, said: "We have never said we were opposed to pathfinder schools trialling it as long as we are in on the evaluation and the teachers' voice comes through.
"What we do not want is for teachers to do the bureaucracy and assistants to have the pleasure of teaching without the bureaucracy. That is hardly likely to reduce teacher shortages, if they end up with more planning rather than less."
Russell Clarke, deputy general secretary of the SHA, said: "It is a matter of principle that teachers teach and assistants help.
"We are opposed to teaching assistants taking classes but it is difficult to say we do not want it trialled because if our views are right then the trialling will indicate that."
An Office for Standards in Education report found that having an assistant could actually create more work for a teacher, because of the extra planning and preparation involved - although it added teachers felt this was worthwhile because the assistant relieved classroom stress.
The "pathfinder" project begins in September and is being evaluated by researchers at Birmingham University, led by Professor Hywel Thomas.