I can understand how a consumer could be confused by how the school zone is determined. You can get street by street information on the Rockville Centre School District website or you can get more information from a local Realtor.
School district areas/lines were set many years ago when the villages were formed. The village of Rockville Centre has almost 30% of it's area going to Oceanside Schools, a percentage to Baldwin and of course the majority to Rockville Centre. Most realtors know the boundaries for these different areas in each town they are listing in. Then of course within each town or area there are further boundary lines for the different elementary schools.