Teachers in England and Wales can no longer be asked routinely to do any administrative or clerical tasks.
Assistants will be expected to take on many jobs
School support staff are now supposed to undertake the tasks removed from teachers' responsibilities.
The change - taking effect on 1 September - is part of the agreement on reducing teachers' workloads being implemented over the next few years.
It was signed by the government and most of the education unions - although notably not the biggest, the NUT.
Its main objection is that in reducing teachers' workloads, more is expected of classroom assistants - meaning, it argues, classes will be taken by people not qualified as teachers.
Originally there were 25 tasks "outlawed" from 1 September, though some have since been lumped together.
The second biggest teachers' union, the NASUWT, is stressing that the list is not meant to be exhaustive but merely gives examples.
It also says the use of the word "routinely" in the contract has been subject to "a great deal of misinterpretation and misinformation".
In copious advice being sent to its members this week, the union says: "The use of 'routinely' is neither to enable heads to require teachers to do any of the tasks occasionally nor to allow teachers to volunteer to continue to do them.
"It is there as a recognition that on rare occasions an exceptional circumstance might arise."
As well as obvious administration such as typing, photocopying and analysing data, the list extends even to such things as classroom displays.
Teachers can devise them, but should no longer have anything to do with preparing them or putting them up.
The union's deputy general secretary, Chris Keates, said: "The agreement heralds a major change in the culture of schools. It will have far-reaching and lasting impact and it should not be dismissed as 'just another new initiative'.
"The agreement will remodel the school workforce and have a profound and positive effect on the way teachers work, bringing real and sustained improvements to their working conditions."
The NUT - National Union of Teachers - says its position remains unchanged.
It notes that the government has changed teachers' working conditions, so its members should not be doing the admin tasks.
But a spokesperson added: "We have a deep concern about the use of classroom assistants to take whole classes and we will not prepare or mark lessons to be delivered in those circumstances."
If any NUT members were disciplined as a result there might ultimately be a ballot for industrial action in the school.
Examples of tasks that should no longer be done routinely by teachers:
- Collecting money from pupils and parents
- Investigating a pupil's absence
- Bulk photocopying
- Typing or making word processed versions or revisions of manuscript material
- Word processing, copying and distributing bulk communications to parents and pupils
- Producing class lists on the basis of information provided by teachers
- Keeping and filing records, including records based on data supplied by teachers
- Preparing, setting up and taking down classroom displays in accordance with decisions taken by teachers
- Producing analyses of attendance figures
- Producing analyses of examination results
- Collating pupil reports
- Administration of work experience (but not selecting placements and supporting pupils by advice or visits)
- Administration of public and internal examinations
- Administration of cover for absent teachers
- Ordering, setting up and maintaining ICT equipment and software
- Ordering supplies and equipment
- Cataloguing, preparing, issuing and maintaining materials and equipment and stocktaking the same
- Taking verbatim notes or producing formal minutes of meetings
- Co-ordinating and submitting bids (for funding, school status and the like) using contributions from teachers and others
- Transferring manual data about pupils not covered by the above into computerised school management systems
- Managing data in school management systems