Fifteen hundred green-and-white-clad PS 321 students marched through Prospect Park on Friday to raise money for the environment.The entire student body of the K-5 school participated in the walkathon, with most of the environmentally conscious tykes wearing coordinated outfits.The march not only raised money for Transportation Alternatives, Amazon Watch, and Added Value -- which operates a farmer’s market in Red Hook -- but got the kids outside on a beautiful day.