Teachers have voted for a campaign of classroom disruption in protest at plans to allow their unqualified assistants to run lessons.
Members of the NUT will refuse to look after groups of more than 27 pupils, subject to a national ballot.
This would mean children being sent home, with the added possibility of strikes taking place.
Delegates at the annual conference in Harrogate, North Yorkshire, were furious at the government plan to let "higher-level" assistants teach.
A motion opposing it was passed on a card vote by 127,597 to 79,355 after a show of hands had been inconclusive.
Jane Nellist, a teacher from Coventry, said: "Our members are desperate for protection and action on workload.
"It will deteriorate further unless we stop it."
The decision comes after a drawn-out dispute between the NUT's executive and left-wing members about tactics.
The executive favoured local action only in areas most affected by the changes, while the left pushed for a national effort.
A show of hands on Monday was too close to call, and the left won comfortably after a paper ballot.
As a result, the executive opposed the amended motion.
General secretary Doug McAvoy said: "It is very difficult to persuade parents that the union is right to do things in order to protect the level and quality of education for their children if we send children home.
"It is not an easy equation to explain to parents."
In January, all the other major education unions signed a workload agreement with the government.
It proposed setting aside extra time for teachers to mark work and prepare lessons.
Certain administrative tasks would no longer be their responsibility.
The agreement was lauded as "historic".
But the 250,000-member NUT maintains that this will not reduce workload and that allowing assistants to teach is a money-saving ploy.
The motion says: "Schools with budget difficulties and acute teacher shortages are likely to be among the first to feel pressure to use support staff as a substitute for qualified teachers, as are those with an emphasis on vocational, rather than academic, 'curriculum pathways'."
It goes on to voice fears of a "two-tier" workforce and education system.
The use of unqualified staff is a "bankrupt" response to problems, it adds.