Picket lines formed at schools across Northern Ireland on Wednesday as around 800 support staff began a 48-hour strike.
Members of Unite the Union - the majority transport workers - are protesting over the ongoing dispute about pay and a grading review.
Dozens of workers gathered outside schools across the north, including Kilcooley PS in Bangor, Lismore College in Craigavon and Glenveagh School in Belfast, throughout the day on Wednesday.
It comes ahead of another three unions, Unison, GMB and Nipsa, joining the strike action on Thursday.
School cooks, cleaners, bus drivers and clerical staff are all due to take part in the industrial action, which is expected to be one of the biggest strikes among non-teaching unions in years.
Unison and GMB members employed by the Education Authority (EA) will walk out for the full day on Thursday, while Nipsa members will strike for two hours after the start of their shifts.
It came as NIPSA said it is planning to "escalate industrial action" further.
The union, which represents over 8,000 members involved work in schools, support services and EA headquarters, has been engaged in action short of strike action since November 6.
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But it said that the strike on Thursday will be "the first escalation of the action" ahead of possible further dates in late November/December.
Alan Law from NIPSA said: ‘There is a real anger amongst education workers who feel that the true value of their contribution to the delivery of education has been ignored for too long.
"They resent being ignored.
"They are right to feel angry.
"The delays in progressing the matters in this dispute which seek to reform their pay structure, make it fit for purpose and future proof it just adds insult to injury.
"The current action has demonstrated just how much EA school based and HQ staff do that is outside their job description.
"This goodwill has ended and won’t be returning."
Unite has said that the strike is the latest development in its "ongoing industrial dispute over the failure to deliver a pay and grading review to education workers as part of a negotiated resolution of the 2022 pay dispute".
It says that recent communication from the Department of Education "confirms that business case for negotiated commitment has still not been submitted".
Members returned a 94% majority in favour of this round of industrial action.
Unite general secretary Sharon Graham said: “It is simply unacceptable that the Department of Education continues to renege on the implementation of a pay and grading review that has been negotiated by the Education Authority with Unite".
Statement by @1977susan in support of the 48-hour strike by school support staff. pic.twitter.com/S52rrbk0I1
— Unite the Union NI (@UniteunionNI) November 15, 2023
The Department of Education last week said it was continuing to liaise with the EA and trade unions over staff pay and grading.
The EA also said steps have been taken to identify the potential impact of the action on schools and services.
