Four of the big unions have called the action in an escalating row over pay.
It has led to classroom assistants, bus drivers, catering staff, cleaners and other support workers walking out at hundreds of schools.
A representative from one union said up to 20,000 are expected to take part.
The Department of Education (DE) said it could not afford to increase wages on its current budget.
Officials said they would need extra funding to do so and were working with the Education Authority (EA) to try to resolve the matter.
The authority said it expected significant disruption on Thursday, as members of three unions - Unison, United and GMB - walked out for the full day.
Nipsa union members are striking for two hours after the start of their shifts.
At St John the Baptist Primary in west Belfast, classroom assistants Eamon McQuillan and Maria Rogan have been taking part in the strike.
Mr McQuillan said he is working two other jobs, one in a youth club and the other at the SSE Arena in Belfast, in order to pay his bills.
He said he needs this extra income to pay for "day-to-day costs" such as helping his parents with bills, paying his for his mobile phone and food.
"That's why we are out here, on strike, looking for better pay," he said.
"We are also out here because a school doesn't run without a classroom assistant, a child's education doesn't run without a classroom assistant, especially in terms of one-to-one."
Meanwhile, Ms Rogan said she was taking the action as she felt she deserved better pay for the work she did.
"They need the help and support from us, if we weren't here they wouldn't have it," she said.
"For them to get the education, we need to be there, to see them succeed in different areas just means the work pays off."
Parent Lynsey O'Donnell, who has two children who receive support at St John the Baptist, said she fully supports the strike.
Ms O'Donnell praised the work of the assistants, who she said were vital in keeping her children in school.
"I had one child in particular that last year was a real, real struggle. We couldn't get him to school, he couldn't stay at school, he was on a reduced timetable," she said.
"He was out of school more than he was in school but this year he has an assistant and he has been in school every single day."
St John the Baptist's principal Chris Donnelly said those in the school community were appreciative of non-teaching staff and "fully behind" the action they have taken.
"We cannot deliver on our mission of educating all children, in developing them academically, socially, emotionally and physically without classroom assistants and without our non-teaching staff," he added.
Anne Taggart, a cook at Holy Child Primary School in west Belfast, said her wage has not changed in 21 years in the job.
She said the cost-of-living crisis was impacting her "ten-fold" and it has got to the situation of "eat or heat".
Ms Taggart added that cooks and catering assistants are leaving in their droves as they "cannot afford to exist".
In Londonderry, staff members outside Ardnashee Special School and College said they felt they had no choice but to strike.
Sharon Thompson, a classroom assistant, said: "We love and idolise these children and we want to be in work today but we have no option but to take a stand for the greater good."
Fellow classroom assistant Kyle Doherty said they needed to be recognised for the work they do with children who have very complex needs.
"I'm here because the job, role and responsibilities that we fulfil everyday are not reflected in our wages," he said.
"We don't want to be here but we feel we have no choice and we need to make a point. People don't actually realise what we do. The demands of the job have changed so much at times you feel like you have to be a teacher, carer and a nurse."
Raymond McFeeters, the principal at Ardnashee, said his staff have the school's support.
He said classroom assistants did "an amazing job" but had been made to feel "undervalued".
Despite the responsibility and often complex work their job entails, many are paid little more than minimum wage, he added.
"One of the big problems is they can earn more money by going to other jobs with much less responsibility and which are much less challenging," he said.
"If we don't support our staff in taking this action, we may lose a lot of staff and then we won't be able to function at all."
At Holy Trinity Primary School in Enniskillen, senior clerical officer Stephanie Hamilton said a pay rise was long overdue.
"We all work very hard, we love our jobs but we're finding it more and more difficult in a cost-of-living crisis to survive," she said.
"So we just want fair pay and to be treated well, and the regrading is really important to us."
Ms Hamilton said she has been encouraged by the support from teaching staff, parents and the public.
"Today we can tell that by standing on the side of the road, them tooting their horns and waving, you can hear them now," she said.
"We hope that it works. We've got to do something and we hope that we get the support that we deserve."
The Department of Education said a pay proposal put forward by the Education Authority and trade unions would have a significant cost.
The initial annual implementation cost for the EA would be Â£39m, with the subsequent recurring cost rising to Â£71m after three years, according to the department.
"However, there is currently no budget available to implement this without further funding being made available."
Thousands of non-teaching staff have gone on strike from Northern Ireland schools with one classroom assistant saying he has to work three jobs to pay his bills.