Brian Carter, the union's West Midlands regional organiser, backed an agreement between Birmingham council and the GMB, the union representing support staff. The deal limits the amount of time teaching assistants can cover for absent staff or teach whole classes.
Higher-level teaching assistants will be limited to taking three sessions per week, while other classroom support staff will only be allowed to cover two hours a week in emergencies.
The deal provides pay increases for support staff and maintains special responsibility allowances, which GMB regional organiser Rob Kelsall said were being eroded by several local education authorities.
Speaking at a conference in Birmingham this week to promote the agreement as a national model, Mr Carter said: "They are talking about a teacher for every class and an assistant for every class. There is a lot of ground where there is agreement.
"We do not want exploitation of support staff, being asked to do tasks for which they are not qualified, trained or paid."
Mr Carter said he still had reservations about unqualified staff taking classes alone, but said the Birmingham deal was a step forward.
He said: "We welcome the limitations written into the Birmingham agreement.
It is not going to be teachers on the cheap. The pay should attract good, qualified and trained support staff."
Martin Hird, GMB's head of public services in the West Midlands, said: "We don't really want to make it a clash between assistants and teachers. I think we have the same fears as the NUT. We will be fighting and campaigning for our members to ensure this agreement is implemented nationwide."
Steve Sinnott, NUT general secretary, said the Birmingham deal would still not be acceptable as a model for the whole country.
He said: "It is still in breach of the fundamental principle that stopped us signing the national agreement. We do not accept that higher-level or classroom assistants should be used to take whole classes. Limiting the amount of time for that does not deal with the problem that a person unqualified to teach is taking a whole class."