Two teenagers from St Michael's College in Dublin have died in separate incidents while on holiday on the Greek island of Ios.
The school described the tragedies "one of the darkest we have ever experienced".
In a letter to parents this evening, Principal Lorna Heslin said the thoughts of staff, families, students and past students are with those affected at this time.
Andrew O Donnell, who turned 18 on 7 June, was on holiday on Ios with classmates after completing his Leaving Certificate exams last month.
He is understood to have gone missing on Friday before his body was located in a rocky area over the weekend.
Today a second student, Max Wall, also died on the Greek island, where he had travelled after completing his Leaving Certificate exams.
The Department of Foreign Affairs is providing consular assistance to both families.
In a letter to parents this evening, Ms Heslin wrote: "In the school's history, there have been difficult days but this is one of the darkest we have ever experienced.
"The deaths of two young men as they get ready to embark on the next stage of life is so difficult to accept.
"The sense of community which is the hallmark of St Michael's will be needed to support the O'Donnell and Wall families, and also the other members of the leaving certificate class as they come to terms with the deaths of Andrew and Max."
Ms Heslin told parents she was writing with "a profound sadness and a sense of great loss" to inform individuals of the deaths of the two young men.
She said Andrew "was a valued member" of the school, and that she wanted to extend "our deepest sympathy to his sorrowing parents and family as they come to terms with the loss of their darling Andrew".
Ms Heslin said teachers and staff also have "many fond memories of Max", describing him as "an outgoing bubbly child, always kind and polite, and enthusiastic, about every aspect of school life".
She said it was "always a pleasure to meet him on the corridors", adding "at this time of unimaginable grief, we extend our deepest sympathy to Max's parents Niall and Fiona, and his brother" who also attended St Michael's.
Minister for Higher Education Simon Harris said the news was "devastating".
He said he is thinking of the two men's family and friends and "all Leaving Cert students abroad in Greece who were enjoying holidays after exams and are now encountering such shock and pain and grief".
In a statement, The Spirtitan Education Trust, said: "Our thoughts are with the parents and families of these two young men who today are suffering immeasurable loss."
"We extend our deepest sympathy and support to their grieving families, to all in their wider circle of friends and fellow classmates who will be mourning their passing and to the Board of Management, the school leaders and all our colleagues on the staff of St Michael's College."
