Students at P.S. 138 in the Bronx unveiled their new park at a ribbon-cutting ceremony Wednesday.Students worked closely with a park design landscaper to design the park that will feature a rock-climbing wall, a butterfly garden and more than a dozen trees.The P.S. 138 playground is the first of 221 playgrounds that will be converted into parks. The parks will be open to the public after school, on school holidays and weekends.