A court has been told a student who admitted the manslaughter of a teenager in the grounds of a school in Enniskillen will live with the consequences for the rest of his life.
He had initially denied the charge.
Oisín McGrath died four days after suffering serious head injuries at St Michael's College in February 2015.
Dungannon Crown Court was told McDermott had been among a number of sixth formers who chased a group of Year 9s when they ran off with a football.
The court heard McDermott had later told police he had hit Oisín on the front of the neck, that it was minimal force and he was very, very sorry and very guilty.
The prosecution counsel Ciaran Murphy QC said a number of Victim Impact Assessments had been submitted by Oisín's parents, grandparents and relatives, in which he was described as a perfect young man.
The judge said it was clear Oisín was a very well-loved son, brother, grandson and nephew.
In mitigation, defence counsel said the defendant did not set out to hurt Oisín McGrath and did not for one moment consider how tragic the consequences of that one punch would be.
The lawyer also told the court McDermott was 17-years-old at the time and that there was no malice or ill-will.
He said this case was exceptional among one-punch cases as it was bereft of any of the aggravating factors found in many others.
The judge said he would need time to consider the submissions.
Sentencing was adjourned until next week.