By Mike Baker BBC education correspondent

They were a cheerful and positive group in the primary school staff-room at break time (and it wasn't even a Friday).

There was coffee and biscuits too.

So I thought it a good moment to test the waters and see what they thought about the "historic agreement" on teachers' workload that was being signed that day.

They suddenly looked less enthusiastic.

The high-ceilinged Victorian former classroom went quiet for a moment. They reflected. Then they said they did not trust it.

"But surely", I asked, "you'd welcome half a day a week free from teaching to do all your preparation, marking and administration, wouldn't you?"

"Yes, of course," they said. But they just did not believe that greater use of classroom assistants, including letting them take charge of a whole class, would actually save them any time.

Grievance

Indeed, they believed planning and organising the work for their assistants would increase, not reduce, their workload.

I reflected on that exchange as I listened to the differing points of view of the teacher unions which had supported the new agreement on teachers' workload and the warnings of the National Union of Teachers which, alone of the main unions, has refused to sign the deal.

At the heart of the deal is an attempt to release teachers from a long list of routine administrative chores, such as photocopying or collecting dinner money, and to ring-fence two and a half hours a week during the school timetable for preparation and marking time.

The wider, and urgent, purpose is to do something about teachers' sense of grievance over workload which, according to opinion surveys, is a major factor in staff leaving the profession and, therefore, a cause of teacher shortages.

These aims, at least, are welcome to all.

Everyone knows that without enough good quality teachers, expert in the subject and age group they are teaching, all other initiatives to raise standards are just so much hot air.

But it is the means to these ends which have created a damaging rift in the teaching profession.

On the one side is the NUT (whose membership is predominantly from primary schools) and on the other are the head teacher organisations, the unions representing classroom assistants, and the other teachers' unions.

Trust

The NUT's position is strongly influenced by its leading role in the long battles of the past to make teaching an all-graduate profession.

Although it is cautious about saying it (for fear of offending classroom assistants) it sees any move to allow anyone but a fully-qualified teacher to take a class as a return to the old monitorial system of the 19th and early 20th centuries.

That may seem a bit far-fetched but so much of this row is about trust.

Quite simply, the NUT does not trust the government or all head teachers not to exploit this agreement to substitute cheaper classroom assistants for expensive (and sometimes hard-to-recruit) teachers.

The other point of view does not dismiss these worries but believes the potential gains outweigh the risks.

It cannot have been easy for some of the other teachers unions, especially the NASUWT, to sign up.

But they believe the deal could actually enhance the professionalism of classroom teachers, giving them a managerial responsibility for assistants and relieving them of mundane, time-consuming chores.

They also recognise that while teacher recruitment and retention remains a problem there is no prospect of improving teachers' working hours without delegating some classroom time to properly trained, and supervised, classroom assistants.

No triumphalism

It is interesting that the greatest suspicion over the plans comes from primary school teachers who are the least likely now to have any time during the school day for preparation and planning.

A few years ago, ministers would have made the most of a disagreement like this between the teaching unions; divide and rule was the motto.

But I have detected no triumphalism over the isolation of the NUT on this.

There should be regret on both sides that government and the entire profession cannot move forward together on the issue.

Parents too should be concerned. This is a difficult dilemma. Of course, in an ideal world parents would wish every class to have a fully-qualified teacher.

But they probably also want a teacher who is not exhausted by an over-long week or distracted by administrative chores.

Above all, in parts of the country where shortages are most acute, they simply want a teacher - and a teacher who will stay and not pack it all in because they are demoralised and worn out by an excessive workload.