Catholic school students in Brooklyn headed back to the classroom Wednesday amidst a wave of school closings.Brooklyn Archdiocese officials say four schools across the region did not reopen this year. Much of those closings can be attributed to rising costs and diminishing enrollment, according to the Archdiocese. Officials say Catholic schools are also having trouble finding and retaining teachers.The problem is not limited to Brooklyn, educational experts say. Across the nation, private religious schools are experiencing similar issues.