Teachers in schools across Belfast and beyond are staging a one-day strike as part of a bitter pay dispute.
Members of the National Association of Schoolmasters Union of Women Teachers (NASUWT) are taking action in around 100 schools in Belfast and Newtownabbey.
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.
Unions have rejected an offer that would have delivered a 1% pay rise this year, with pay rates for 2015/16 remaining frozen.
Stormont's Education Minister Peter Weir has claimed a bigger increase could only be afforded by making redundancies.
He has said he was prepared to discuss "realistic" pay proposals for 2017/18 onward 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.
Ahead of the action, the minister urged unions to call off what he branded a "futile" strike.
"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 teachers the worst off in the UK.
NASUWT general secretary Chris Keates said: "It is with deep regret that we have been forced to move to this position, but we have been left with no choice by the blatant disregard of the Minister for Education for the pay and conditions of service of teachers who provide such a vital public service."