But mass strike action is being resisted by union leaders who want to keep parents and governors on their side in their protest campaign against the Government's funding of schools.
They fear advantages gained through the recent alliance of schools, governors, parents and opposition politicians could be lost if children's education suffers.
Politicians and heads' organisations have already united in condemnation of any industrial action by teachers, while parents' leader Margaret Morrissey has warned that support for teachers is "hanging on by the skin of its teeth. "
The National Union of Teachers' conference decision to ballot on a national one-day strike has split the unions on how to tackle the issue.
The outcome will be three separate strategies operating in schools, with action likely to drag on from the summer through the autumn term to next January.
Action against the NUT's proposed strike is led by the union's own general secretary, Doug McAvoy, who believes it is not the answer to the problem of increasing class sizes and hopes the wider membership will vote it out.
Mick Carney, National Association of Schoolmasters Union of Women Teachers treasurer, told the annual conference meeting in Eastbourne that a strike was a weapon of last resort. NASUWT intends to have a consultative ballot of members next term and is expected to take action in the autumn term in schools experiencing problems.
It has not defined maximum class sizes, saying the type of class, age of children and other factors should be considered. Any action - most NASUWT members work in secondary schools where the class size problem is not acute - is likely to be reviewed next January when the report of the School Teachers' Review Body on pay comes out. NASUWT wants the document to refer to class sizes.
The Association of Teachers and Lecturers voted to advise members not to teach classes of more than 31 (29 in mixed-age classes) for more than two consecutive days. Deputy general secretary Gerald Imison said this could lead to partial or occasional exclusion of children. If a solution cannot be found strike ballots will be held.
He said: "While the mood is to do something, our members are realistic and are concerned that strike action will not achieve the aim of reducing class size."
Any action is likely to be spread disproportionately between different regions and school sectors. NUT officers expect an escalation of strike action in schools in areas where classes are large: Sheffield, Gloucestershire, Devon, Nottinghamshire and Staffordshire have already held strikes over cuts and could be worst hit as "class-size blackspots".
Primary schools, where the NUT has the most members, are in the frontline of the class size battle and ballots have already been held. Union leaders said there were 20,000 primary school children in classes of more than 40 and one million in classes of more than 30.
Colin Tarrant, NUT executive member from Derbyshire, will ask schools whether they want to take action on class size. If the matter cannot be worked out within schools, he expects strike ballots will be called.
"The teacher unions may have expressed their concern differently at the conferences, but there is no doubt of the uninamity on how seriously all three are taking the matter," he said.
David Hart, general secretary of the National Association of Head Teachers, said: "I don't believe you can march arm in arm with parents in support of a funding campaign and then turn round and say you are sending their children home." But he conceded that if teachers refuse to take classes over a certain number heads will be forced to exclude pupils on a rota basis.
Russell Clarke, assistant general secretary of the Secondary Heads Association, was disappointed by the conference decisions which he feared would break up the alliance between parents, governors and teachers.
"I am concerned about the legal implications of teachers excluding children, " he said.
Education Secretary Gillian Shephard stressed class size was not a "black-and-white issue". Warning teachers at Eastbourne that parents did not want disruption in schools, she added: "After this week the public might need reassurances that our faith in teachers' professionalism remains justified. "