In addition, three schools on Suffolk's East End and two in Nassau will be asked to engage in "collaborative" efforts to work together and strengthen their programs if they want to remain open long-term, said Sean Dolan, a spokesman for the diocese.
"Closing schools is one of the most painful parts of my ministry," Bishop William Murphy said in an open letter posted on the diocese's website. "My goal was to make decisions now that will allow us to say that there will be no more school closings beyond my time as your bishop."
The schools shutting their doors in Nassau are: St. John Baptist De LaSalle Regional School in Farmingdale, St. Catherine of Sienna School in Franklin Square, St. Ignatius Loyola School in Hicksville and Sacred Heart School in North Merrick.
"It is with a heavy heart that I write to inform you of the closing of Prince of Peace Regional Catholic School at the end of the current school year," Principal Jane F. Harrigan said in a message posted on the school's website. "At this sad time for all of us, it is comforting to know that the spirit that is Prince of Peace will carry us through the year."
The schools in Suffolk instructed to work collaboratively to stay open in the long term are St. Isidore School in Riverhead, Our Lady of Mercy Regional School in Cutchogue, and Our Lady Queen of Apostles Regional School in Center Moriches.
Dolan said a combination of declining numbers of grammar-school-age students on Long Island, predictions of more declines in coming years and the difficult economy forced the diocese to implement the plan. It is the largest number of diocese school closings at once in two decades.
"Many public school districts face the same reality: They just don't need as many school buildings as they have in the past. That's the case here," Dolan said.
The reorganization will drop the number of grammar schools from 53 to 47. The historical high in the diocese was 92 in the 1960s, when there were 78,000 students.
Enrollment in diocesan grammar schools has dropped 34 percent in the past decade, from 28,709 in 2000-01 to 19,261 in 2010-11, Dolan said. In addition, the grammar-school-age population on Long Island is predicted to drop 7.5 percent by 2015, diocesan officials say.
The diocese had bucked a national trend for about five years when no grammar schools closed. But four shut their doors over the past two years. Church officials said they wanted to avoid repeated instances of single closings and reorganize their system with one comprehensive move.
Dolan said the hope is that the diocese will not have to carry out any more closings for the rest of Murphy's tenure as bishop here, expected to last four or five more years.
"Obviously, you don't know what tomorrow brings," he said. But "with this reorganization, we should be in good shape for the future."