That's usually the way it works. The mission of your existing PTO is to help make that one school better. But you want to create a PTO with a different mission, one that would bring together parents of special needs children from across the district. If your current PTO were townwide and supported several schools, you might create a special needs committee under that umbrella. But in your case the special needs group would be serving schools not served by the current PTO. Thus, you would start a separate PTO.