The subject sparked passionate speeches at the annual conference of the National Association of Head Teachers (NAHT) and led some members to heckle Beverley Hughes, the children's minister.
Mick Brookes, NAHT's general secretary, said he hoped other teachers' unions and parent groups would join him in creating a plan of action against key stage tests. He called on ministers in England to follow their counterparts in Northern Ireland and issue a legal injunction blocking newspapers from publishing league tables.
Mr Brookes warned of "trouble ahead" and refused to rule out a boycott. "If there is still this obstinacy, we will have to consider our options by November," he said.
Concern over testing outshone all other issues at the conference in Liverpool, which attracted more than 500 school leaders.
The association voted unanimously for a series of motions pressing for changes to assessment. These include use of other measures to hold schools to account and the introduction of more realistic timescales for curriculum reforms.
Eric Fisk, deputy head of Tanfield School in County Durham, said schools were being held accountable through an "unholy trinity of targets, tables and tests".
He warned that the new when-ready tests, which KS2 and 3 pupils can sit in December or June, would not necessarily improve matters.
"If they become nothing more than Sats twice a year, then any opportunity for creativity will be stifled," Mr Fisk said.
Delegates also backed a motion opposing no-notice Ofsted inspections and league tables.
Simon Springett, a former Hertfordshire head, said: "We have no fear of fair and accurate information. But we have no time for misleading and detrimental data that undermines the achievement of pupils and undermines the achievements of schools."
After Ed Balls, the Children, Schools and Families Secretary, appeared only on a pre-recorded video, speakers criticised the "virtual secretary of state".
The headteachers politely applauded a speech by Mrs Hughes, but several heckled "Rubbish!" and "You do the job!" when she defended leagues tables during a question-and-answer session.
Ms Hughes gave critics of the primary curriculum some hope when she suggested it had become too prescriptive, and said that she wanted Sir Jim Rose's review to address that. "The balance has gone too far," she said.
The other main bone of contention was the responsibility increasingly being placed on heads for children's development outside schools. Alan Norgrove, head of St Mary's Junior School in Surrey, said: "Very shortly, the only bit of child-rearing we won't be doing is the conceiving of the child."
Mr Brookes said heads supported after-school activities but feared a minority of parents were too reliant on them as a childcare service.
Clarissa Williams, NAHT's president, said she had written to the Prime Minister suggesting rewards for parents who spend quality time with their children and support their schools.
WHO SAID WHAT ABOUT TESTING
Chris Howard, head of the Lewis School, Caerphilly: "We shouldn't be measuring progress to account to parents; we should be measuring progress to help the child do the best they can."
Clarissa Williams, NAHT president: "The assessment machine leads to a mechanistic model of learning for too much of the time."
Roger McGough, poet, (left): "I don't like league tables. I find them very falsified. Competition is OK when it's a 100 yard race, but not this."
Rona Tutt, special educational needs consultant: "Until all schools have identical intakes, tests should not be used to hold schools to account."
Mick Brookes, NAHT general secretary: "The department says children are only tested 0.2 per cent of the time. This is rather like saying that the corpse was only shot once."
Beverley Hughes, Children's Minister: "Look, the views and opinions of headteachers and teachers in this area are very important, but it's not the only perspective that's important - there's also the perspective of parents."