Head teachers are grilling politicians from the three main parties on their plans for England's education system.
The heads are due to boycott national tests in England for 11-year-olds.
Schools Secretary Ed Balls, who was greeted with polite applause, told head teachers "we have not agreed on everything this year".
Conservative Michael Gove and David Laws of the Lib Dems are also appearing at the National Association of Head Teachers (NAHT) conference.
Mr Balls, the first of the three politicians to speak at the conference in Liverpool, urged delegates not to go ahead with the boycott, saying that would not be fair on children or parents.
Mr Gove was applauded for saying teachers and heads should be empowered and Ofsted inspections overhauled.
And Mr Laws was also applauded when he said there was an absurd level of political meddling in schools. His comments that schools were in danger of becoming "desiccated exams factories" also drew approval from the conference floor.
Whichever party wins the election, its leaders will face the challenge of dealing with this rebellion by head teachers.
Normally a very moderate group, it will be the first time in a quarter of century that the NAHT has taken industrial action.
The tests, in English and maths, are due to be taken by about 600,000 children in England from 10 May onwards.
On Saturday, the heads were addressed at their conference by National Union of Teachers general secretary Christine Blower, who said about half of England's 17,000 primary schools would boycott the tests, which are known as Sats.
The NUT and the NAHT are taking unprecedented joint action against the tests, the results of which are used to produce the primary school league tables.
They say the tests are bad for children and for schools.
'Not set in stone'
Teachers, they say, focus their efforts in the final years of primary school on getting children to shine in the tests and teach a narrow range of subjects.
And the league tables, they say, have a corrosive effect on schools and do not reflect their true achievements.
All the main parties are against the boycott. The Conservatives and the Liberal Democrats both say they would reform the national tests, with the Lib Dems promising greater use of teacher assessment.
Schools Secretary Ed Balls has said that the tests help to drive up standards and provide parents with vital information about their children's progress and their local schools.
He has also said that the tests are not "set in stone".
England is the only part of the UK to have these externally-marked national tests.