"Our first priority is the safety of our students and we are extremely disappointed at the actions of this former staff member.
"We are confident through our investigation that the incident in 2015 was isolated to one student who was then enrolled at King's,'' Mr Reddiex said.
Responding to further questions, Mr Reddiex said the pupil was 18 at the time of the relationship and in his final year, last year.
He was reluctant to share further details about the pupil, but said he had been offered counselling.
"I met with the student and with some extended family members and he has been offered support.''
Mr Reddiex had heard rumours that nude photos of the teacher had been shared on social media, but those rumours had not been substantiated.
The Education Council (formerly the Teachers Council), which investigates and rules on teacher misconduct, was informed about the relationship.
Board of trustees chairman David Booth said he was not fully aware of the facts of the case, but the school was taking the situation "extremely seriously''.
"We expect a high level of professionalism from all our staff and board members and it's extremely disappointing if this has grounding,'' he said.
