They have condemned government guidelines which say that assistants could be expected to take whole classes. The National Union of Teachers said the proposals were redolent of the Tories' Mum's Army idea.
The guidance, still under consultation, says confident classroom assistants should "address the whole class for a time according to plans made in advance with the teacher". It also suggests teachers delegate tasks, such as assessing pupils' literacy and numeracy performance.
Doug McAvoy, NUT general secretary, said: "The Department for Education and Employment is using examples that denigrate and confuse the role of the teacher."
The union also criticised the guidelies for implying that classroom assistants can undertake work carried out by special education needs co ordinators and specialist language support staff.
Between 1992 and 1998 the number of support staff rose to 59,000 - a 60 per cent increase. In that period the number of teachers rose by just 2 per cent.
The Government aims to recruit another 20,000 assistants by 2001.
Nigel de Gruchy, general secretary of the National Association of Schoolmasters Union of Women Teachers, said: "Classroom assistants are rapidly becoming teachers in practice."
Pay levels for classroom assistants are among the lowest of any staff group in local government, ranging from about pound;9,000 to pound;12,000 a year.
The guidance pre-empts a national framework of training and qualifications for classroom assistants.