Headteachers could introduce a four-and-a-half day week as a "last resort" amid a growing funding crisis.
School leaders could also consider refusing to submit their budgets, or run a deficit budget, it was suggested.
Delegates at the National Association of Head Teachers (NAHT) annual conference backed a call for its executive to "explore every available option open to schools" if "irresponsible funding cuts" are not reversed.
"All options should be considered, not excluding: reducing the school week to four and a half days; refusing to submit budgets to local authorities or EFA; permitting all schools to run a deficit budget," the amendment to a motion on funding cuts said.
Ministers have previously insisted that school funding is at record levels, and this will increase further over the next two years as pupil numbers rise.
Speaking after the debate, Carlisle headteacher Clem Coady, who proposed the amendment said: "We feel, when we put this amendment together, that the four-and-a-half day week must be seen as ultimately, the very last resort because we don't want to cut the offer that we are giving to parents, to children, to families, to our staff.
"But there's got to be some way of forcing and opposing these Government-imposed cuts."
These cuts are unworkable and must be overturned, he said.
Fellow Carlisle headteacher Graham Frost said that no school leader wants to reduce what they offer, cut their curriculum, or staff numbers and that heads have already "trimmed our sails".
They are also confident that "the parent voice is so strong that it will win through and that no government is going to be able to ignore such a loud voice across the country."
"We are confident that, come the autumn statement, this £3 billion shortfall in the schools budget will be reversed," he said. "But, in the meantime we've got to prepare for any eventuality."
Mr Frost told journalists: "We want properly funded schools and we want the Government to stop peddling this denial.
"It's driving us crazy that we seem to be getting nothing but a recorded message back any time anyone questions this crazy assertion that schools have got lots of money," he added.
Mr Frost said the motion authorises the national executive to keep school funding as a priority, up to, and then beyond the election.
"We've got in our sights that autumn statement. That's the next opportunity of any government to make the changes that we are demanding."
The resolution, including the amendment, was passed by delegates at the Telford conference.