I can confirm that. In fact, both public elementary schools and public middle schools are numbered here. The designation for the former (which start with kindergarten and typically go through grade 5, sometimes higher) is P (for Primary); the second sort usually cover grades 6 through 8. Both P.S. 87 and M.S. 245 (periods are used, with a space before the number) are near my home.I believe that in New York some elementary schools (schools for younger children) are known by numbers, such as PS 43, [=Public School 43]. In this case, as I understand it, people say the letters and the numbers.