Teachers staging a one-day strike over pay, workloads and job security have said they had no choice but to walk out.
Members of the National Association of Schoolmasters Union of Women Teachers (NASUWT) are taking action in around 100 schools in Belfast and Newtownabbey.
Hundreds turned out for a rally in Belfast's Europa Hotel.
Jane McConville, a teacher at Elmgrove Primary in inner east Belfast, said: "I am not someone who strikes lightly.
"It is something that I have thought long and hard about, but I feel at this stage we need to support the Union's directive.
"We need to stand up because I feel that the Government is not interested in our stresses; in what we are going through; that if we don't stand up now it will be a case of what will they come for next."
Unions have rejected an offer that would have delivered a 1% pay rise this year, with pay rates for 2015/16 remaining frozen.
Other teaching unions are currently taking industrial action stopping short of a walkout. However, they plan to ballot members on escalating their protest to a strike.
Stormont's Education Minister Peter Weir has described the strike as "futile" and claimed a bigger increase could only be afforded by making redundancies.
Ms McConville added: "I think he is trying to create a lot of smoke and mirrors.
"It is a fact that the money was allocated to give the teachers their 1% pay rise. The teachers in the rest of the UK have had it.
"For him to describe it as futile is very disingenuous.
"He is not listening to the teachers. Not only are we teachers, we are also the electorate and I think he would need to actually take notice."
Mr Weir has said he was prepared to discuss "realistic" pay proposals for 2017/18 onwards but said there would be no more negotiations on the rejected offer.
Mr Weir said while the majority of impacted schools would remain open on Wednesday, some would only be open for staff.
"It will cause major disruption to parents and put further pressure on other teachers and leaders that are already struggling to cope, in addition to damaging the reputation of teachers," he said. "The negotiations are over."
The union claims years of pay restraints combined with increased pension and National Insurance contributions, all amid rising inflation, have left Northern Ireland's teachers the worst off in the UK.
Lawrence Watson, a teacher at Christian Brothers' School on Belfast's Glen Road, said: "This is about the future of education.
"It was an incredibly difficult decision to strike.
"I don't want to be here; I don't want to be on strike; I want to be with the kids; I want my son to be in school and for teachers to be teachers.
"The problem is we are not being allowed to be teachers at the moment and we don't have recognition of what we are being asked to do.
"If our conditions are being changed there needs to be parity with the pay.
"The minister would be very quick to say we have the best education system in the world, well he needs to assess the inputs into that and that's the teachers - you have the best teachers in the world; the best young people in the world and he needs to realise he has a problem, not us."
Meanwhile, Justin McCamphill, NASUWT Northern Ireland official, said morale had hit a new low leaving teachers with no alternative to strike.
He said: "Over the last seven, eight years the workforce has diminished by at least 10%.
"The work in the classroom, however, still needs done so people are doing more work than they were in the past; sickness levels are rising and that is a direct result of the pressures people are now under.
"Teachers don't want to strike but they feel they have no other option left."