It’s an adolescent rite of passage. Every spring, high school students gather at the prom to dance and circle each other awkwardly. A few years ago, the May Center, a nonprofit special-education school south of Boston, created a prom just for kids with autism and other developmental disabilities. Like their peers at traditional schools, the teens at this prom want to look good and have fun. And they can be pretty strong-willed about what they do — and don’t — want to do.