A single pay negotiating body should cover all one million teachers and school support staff in England and Wales, union members will demand next week.
Unison, which represents nearly half of the support staff in schools, is expected to call for a single pay group at its annual conference in Bournemouth.
The Government has already agreed to a negotiating body that would decide the pay of an estimated 500,000 support staff.
Unison believes that expanding the body to included teachers would increase its clout.
Last week, the body for National Health Service workers, which represents a million nurses and hospital staff but not doctors, won a 2.75 per cent pay rise. That is higher than the 2.45 per cent offered to teachers and other school staff.
Christina McAnea, Unison's head of education, said the risk of a joint schools negotiating body was that support staff would be seen as second-class citizens, after teachers. But, she said, the combined strength of one million school workers counted in its favour.
"School support staff are among the worst paid in the public sector," she said. "They have taken on increasingly complex roles, but their pay hasn't kept pace. Schools can't rely on that goodwill forever."
The National Union of Teachers is not in favour of a single pay body, but has joined Unison in calling for a united campaign for better pay for school staff.
Hundreds of teachers, support staff and other public sector workers attended a rally in Westminster this week to lobby MPs for an above-inflation salary increase.
Christine Blower, NUT acting general secretary, told the rally: "It's not that public service workers want more money - they need it."
Unison and other school support staff unions may strike again on July 10 and 11, with the support of other workers in local government, the civil services and FE colleges. But NUT teachers will not walk out again before the autumn.
Miranda Lagab, a teaching assistant at Robert Blair Primary in Islington, north London, said she had been unable to send her own children on school trips because she took home just pound;860 a month, despite working a five-day week.
She survives by topping up her income with family tax credit and child benefit, and has just pound;70 per month left after bills and basics.
"The job is not about sharpening pencils," she said. "I support the idea of a strike, as the pay deal for us is an insult. They are playing on our good nature and depending on the fact that many of us are mothers who don't have choice."