Three out of four primary and secondary schools in Scotland face being closed during three days of strikes by non-teaching staff later this month.
The Unison union said 21,000 of its members working across 24 local authorities will walk out from Tuesday 26 to Thursday 28 September.
Unite members in 11 council areas will also strike.
GMB members including janitors and cleaners earlier called off strikes which were due to take place next week.
Unions have rejected an average pay increase of 5.5% - with a 99p-an-hour rise in the living wage for the lowest paid.
Council body Cosla told BBC Scotland News on Wednesday that it had taken part in "constructive" talks with the GMB.
However, Unison said its members remained "steadfast in their resolve to fight for fair pay".
Unison spokesman Mark Ferguson said: "Cosla's offer falls short of Unison's pay claim, it is also less than the offer made to the lowest-paid local government staff south of the border.
"No-one wants to see schools close but Cosla need to come back with a significantly improved pay offer very soon if strike action is to be avoided.
