A powerful government-led group has urged schools to stop paying teaching assistants as little as 15p an hour extra to cover lessons.
The move follows revelations in The TES that the new grade of teaching assistants are only being paid more for time they spend standing in front of a class, rather than a higher salary.
Guidance sent to schools warns headteachers against only paying by the hour as the practice risks jeopardising workforce reform. It says the "casual arrangements" are not in line with the workforce deal. Up to 14,000 higher-level teaching assistants are expected to be trained by September to give teachers at least one half-day a week outside the classroom for marking and preparation.
They are seen as central to the success of the workforce agreement. Unison, the biggest school support staff union, said some assistants receive just Pounds 3.50 an hour more for taking lessons, although The TES found evidence of as little as 15p extra being paid.
Now the workforce agreement monitoring group, drawn from the Department for Education and Skills, Welsh assembly and most teacher unions, has said in its guidance: "This approach risks undermining the professional status of those meeting the HLTA standards."
It says that higher-level assistants will only be effective if their pay properly rewards their new responsibilities - and warns schools against only paying them extra by the hour.
Bruni de la Motte, a Unison education officer, said: "The extra responsibilities put on the shoulders of HLTAs should be reflected in their pay - and not just for the hour or so that they spend covering a class.
Class cover is one of only 31 tasks HLTAs are supposed to be trained to do."
At present, individual schools and local councils draw up payscales for support staff. Unison has already told teaching assistants to refuse to cover classes when schools attempt to pay by the hour, jeopardising the chances of teachers getting their half-day a week of marking time by September.
The guidance from the monitoring group came as it emerged that higher-level assistants were likely to be asked to play a more prominent role the classroom.
The Teacher Training Agency, which became responsible for the in-service training of teaching assistants this year, said that new programmes will be developed to train a new breed of assistants who would specialise in particular subjects.
In a report, backed by unions and other education quangos, the agency recommends the creation of specialist higher-level assistants in subjects including maths and science, to give expert help to teachers.
Nick Brook, from the TTA, said that 400 higher-level assistants would start pilot courses in September to reach A-level standard in maths and science.
Courses in other subjects, including English, could follow next year.
The agency's interim plan for support staff also recommends that some get special training to support pupils who speak English as a second language.
Courses in information and communications technology and how to control badly behaved pupils should also be created for assistants, it said.
The main union representing assistants welcomed the agency's plan. "Any additional training is welcome if it is going to raise the status of teaching assistants," said Ms de la Motte.
"We would support, in principle, the creation of specialist teaching assistants in certain subjects, we don't see that as a threat or unnecessary burden to our members.
"But pay must match any extra responsibilities."