Unison and the GMB unions said about 800 support staff were due to begin two days of industrial action in a dispute over pay.
They say Brighton and Hove council is cutting the number of weeks for which staff are paid to fund a promised pay rise.
At a rally of school staff, Dave Prentis, Unison general secretary, said:
"This penny-pinching attitude towards valuable school staff is disgraceful.
The council must think again and settle this dispute.
"We don't want to disrupt education, and we don't want to inconvenience parents, but the days when school staff worked for a pittance are gone."
Unison said this is the first time teaching assistants have gone on a unilateral strike.
Staff had been offered a one-off bonus payment of pound;450 before Christmas to settle the dispute, but unions said they rejected the "bribe".
At the same time as implementing pay rises as part of the national single-status agreement, the council plans to cut the number of weeks for which staff are paid from 49 to 44.
Striking teaching assistants say that will slash their promised pay rises and breaks the spirit of the deal.
Charles Harrity, regional organiser for the GMB, said: "Teaching assistants are very angry: they feel outraged that they are basically being asked to fund their own pay rise. The council is trying to give with one hand and take away with the other."
He estimated that about half the city's 78 schools would close, while the council said the majority would have to shut.
The council has sent out letters to all teaching assistants in an 11th-hour bid to persuade them not to strike.
It claims 90 per cent of staff would still be better off, and no one would suffer a pay cut. An extra pound;1.7 million is being put into wages for the 1,000 teaching assistants over the next three years, the council says.
Alan McCarthy, council chief executive, said: "We know that feelings are running high, and we all agree teaching assistants do a brilliant job. But we do not believe that inconveniencing thousands of parents, forcing them to take time off work to look after their children, will solve anything."