Schools across Scotland face weeks of closures and disruption after a third union rejected a new pay offer.
Unite has joined Unison and the GMB in turning down the proposal from council body Cosla. All three have warned time is running out to avert strike action.
Parent groups have raised fears school closures will have a "detrimental" effect on their children.
But on Thursday Cosla said there was no more money available for pay without cuts to jobs and services.
The latest offer from the body, which represents Scotland's 32 councils, is a two-part plan which it said would give workers at least a £1,929 increase in annual salary by 1 January 2024.
The decision to reject this means three out of four schools could be closed by a programme of strikes beginning with a three-day walkout on 26, 27 and 28 September.
On Friday Unite general secretary Sharon Graham said: "It has taken Cosla five months to increase their offer by a measly 38 pence a week for the lowest paid council workers.
"Unite's local government representatives rightly rejected this offer. The fight for better jobs, pay and conditions in local government goes on, and if needs be by strike action. Unite will back its members all the way."