Teachers will walk off the job while South Australian students sit year 12 exams, leading to calls the union has broken its word.
The Australian Education Union (AEU) on Monday evening rejected a revised pay proposal from the state government after setting a deadline for Education Minister Blair Boyer to provide an "improved offer" to avert Thursday's industrial action.
The revised deal would give teachers a four per cent pay rise in the first year, followed by increases of three per cent and 2.5 per cent in the following two years, compared to the previous offer of three per cent each year over three years.
By front-loading the increase, compound interest would make teachers substantially better off in the long term, Mr Boyer said.
But AEU SA branch president Andrew Gohl said the offer was the same as, if not worse, than the last.
"Since taking industrial action in September, we have been negotiating with the government and expected today's offer to reflect those discussions," he said on Monday night.
"It is disappointing to see such contempt."
Under the terms of the latest offer, which would cost the state budget $1.4 billion over the life of the agreement, teachers would no longer receive two $1500 one-off payments, which means many educators would be worse off than the previous offer, Mr Gohl said.
"If we don't get this right, not only will we continue to lose our current workforce but fail to attract the next generation of educators as well," he said.
South Australian teachers are some of the lowest paid in the country. Earlier this year their NSW counterparts won an historic wage increase, including a rise of more than 12 per cent for graduates.
The union also took umbrage the government's refusal to bring down the seven-year delay for a reduction in teaching hours, arguing teachers are in desperate need of workload relief and cannot afford to wait that long.
Treasurer Stephen Mullighan said it was disappointing Mr Gohl did not honour the terms of his ultimatum.
"He wants to go through with the strike that nobody in the South Australian community wants except him and his executive," he told ABC Radio on Tuesday.
"Mr Gohl needs to stay true to his word and he needs to continue negotiating in good faith.
"Using school students and teachers in that way is extremely disappointing."
Mr Mulighan said the deal got the balance right between rewarding teachers for their extraordinary work and ensuring the state budget can still afford other essential services.
Hundreds of schools will close or have reduced hours, similar to an earlier strike in September, but Mr Boyer reassured parents Year 12 exams would go ahead at their schools, regardless of whether they were open or not.
He said he was shocked by the union's refusal to call off the strike, which he called an "unnecessary distraction" for the 1000-odd students taking physics and accounting exams on Thursday.