The sudden closure of St Michael's Christian Brothers School in Dublin was inevitable as the school had come to the "end of its natural life cycle", a spokesman for the CBS's education office said.
Conor O'Brien, director of education for St Helen's province, said "the issue of closure has never been off the agenda for the last eight years".
St Michael's CBS in Inchicore, which has been educating boys since the 1930s, will close at the end of the school year.
Mr O'Brien said numbers at the school had been falling to a point where now there were 63 pupils, and this was not a "critical mass" that enabled the school to function adequately.
Next year there would be no administrative principal and no caretaker, and numbers at the school meant it was not getting resources to give "adequate provision for the 63 pupils next year. The trustees are charged with providing an education." They had to come to the decision, he said, "that the pupils' best interests would be best served at another site".
The Department of Education has no decision-making role in the school.
Parents collecting their children from the school yesterday said they were "shocked" at the news. The school is in a designated disadvantaged area and has a high proportion of pupils with behavioural problems and special needs.
The parents said they were "very worried" about whether they would be able to get their sons into schools in the area. Many learned about the closure, due in June, on radio news programmes yesterday morning and were not told formally by the school until yesterday afternoon.
"I'm just totally shocked," said Jean Delaney, whose son, Colm, is in third class. "To be told about it only at 2 o'clock on Tuesday afternoon, when you've only heard rumours from the radio. The child is in third class and it will be awful messy now trying to get him in somewhere else. He has another three years to do and, yes, I am very worried."
She thought it would be difficult getting him into a school in the area as she would be competing with the parents of up to 50 other St Michael's pupils in the same position.
Another mother, who did not want to be named, said she was "furious and raging about this".
"You think they would have had the common decency to give us a bit more notice - a year at least. How can we be expected to all get our kids into a school round here? We won't."
A spokeswoman for the National Education and Welfare Board (NEWB) - which is charged with ensuring children go to school - said the impact of the closure would be "a priority".
"The NEWB educational welfare officer assigned to St Michael's will be available to support parents who are experiencing difficulty in finding a school place for their child," she said.
Asked about plans for the school building, which is the former Richmond Barracks and a protected structure, Mr O'Brien refuted claims that the site would be sold to a property developer.
"There are no plans for the building. It is a listed building," he said.
A number of politicians have called for a review of the decision to close the school, given the projected increase in population in the area. Gay Mitchell, MEP and former pupil, called on Minister for Education Mary Hanafin to establish a review body to "urgently report on the future education needs of Inchicore".
