A teacher at a County Antrim school has apologised for putting up a display using the hash tag 'Darkest Day' on a school notice board.
The words appeared on Dalriada grammar school's Christian Union (CU) notice board on Monday evening.
Dalriada principal Tom Skelton told BBC News NI an investigation into a breach of school policy was taking place.
The school is a voluntary grammar in Ballymoney with over 800 pupils.
Hundreds of former pupils put their names to a letter sent to the school expressing "outrage" after pictures of the notice board appeared on social media.
"Regardless of any individual's personal position on the issues of gay marriage and abortion, it is completely unacceptable that such a display was posted in a school environment," the letter said.
"Dalriada should be a place where all of its pupils, staff and visitors feel welcomed, supported and able to learn, regardless of their sexuality, political views, medical history or religious beliefs."
Mr Skelton said that he had removed the display when he became aware of it on Tuesday morning.
"I just want to make it clear that it was an individual member of staff involved with the CU [who] put the display up, but it was not the decision of CU," he said.
"It did not represent CU, which was the issue, and it also appeared on a CU Instagram page, which was unfortunate as well."
Mr Skelton said a number of Christian Union members were also upset about the issue.
"It had not got the blessing of myself or senior staff in school here," he said, adding that an investigation was ongoing into a breach of school policy.
"The member of staff involved has apologised profusely for what happened, and she has apologised for any hurt or offence it has caused to any other pupils or member of staff," he said.
"A genuine mistake has been made here."
"Every teacher and every pupil is entitled to their views, but that view can't be a public view that affects the care and wellbeing of other pupils in Dalriada."
Mr Skelton said that all pupils had been made aware of the situation at school assemblies on Wednesday morning, and he had also held meetings with the school's staff.
Parents have also been informed of the incident.
'Job of work to do'
He also said the school would reply to the open letter from former pupils and were taking the situation seriously.
"Those are pupils who have been in our school for seven years over a period of time, and I was disappointed that some of them seem to think the school has changed significantly - it's still the same school it was then," he said.
"We maybe have a job of work to do to reassure people but I'm quite happy to do that."
The school's critical incident team and governors are to meet to discuss the incident on Wednesday evening, but Mr Skelton said that Dalriada's ethos would not change.
"Our ethos is for all pupils to feel safe, valued and respected, regardless of age, sexuality, political views or religious belief," he said.
"The school pastoral programme is also being reviewed to ensure that lessons on topics such as same-sex marriage and abortion are delivered in an age-appropriate, sensitive, non-judgemental manner which reflects the views of all."