Many schools in England already give teachers time out of the classroom - as called for under the government's "workload reforms", unions say.
Teachers should not do tasks such as photocopying under the deal
Two teachers' unions have attacked a decision by head teachers to pull out of a deal bringing in the changes.
They say many schools have already made the changes, which give teachers time out of class to prepare lessons.
The National Association of Head Teachers has voted to disregard the plans, which become law in September.
It says schools do not have enough money to implement the reforms.
Chris Keates, the general secretary of the NASUWT, said: "The law is the law and, whatever some NAHT members may think, they are not above it.
"Since time immemorial there have always been some head teachers who have sought to block improvements to working conditions with threats of job loss.
"This is the last desperate refuge of ineffective management.
"Any head teacher who seeks to deprive teachers of their full contractual entitlements should now be prepared to face legal challenge and industrial action."
Another union - the Association of Teachers and Lecturers (ATL) - said many schools had already brought in the changes.
General secretary Mary Bousted said: "This decision represents a sad failure of leadership by the NAHT. It has never sold the advantages of partnership even though most heads are busy implementing the national agreement."
The other big teaching union - the NUT - never signed up to the workload agreement with the government, because it maintains only professionally trained teachers should take classes.
Under the arrangements, classes may be led by support staff, although they should be planned and supervised by teachers.
ATL's branch secretary for Somerset, Andy Ballard, advises on work-force remodelling.
He said many schools in Somerset had come up with creative ways of giving teachers time out of the classroom while broadening the curriculum too.
"I've been working with a group of about 30 schools and a significant proportion have made terrific headway.
"They are making innovative changes which have led to increased flexibility.
"A few have implemented the agreement ahead of schedule and within budget."
One way in which schools were creating the time out for children was to introduce the idea of "golden time", where children do activities not tied to the curriculum.
For example, one had arranged for qualified sports coaches to run lessons with support staff.
Another had brought in members of the University of the Third Age to talk to the children or do activities with them, which had included everything from lace-making and basket-weaving to gardening.
Most primary schools on South Tyneside now have plans to implement the deal, the local authority said.
One head teacher said: "It hasn't been easy but we have used our very good support staff and external sports coaching.
"The staff have benefited greatly from having quality planning time knowing that the children are receiving quality provision. There are still some things we need to iron out and sustainability might be an issue for us."