I claim the ornamental design for grade board skirt, as shown.
FIG. 1 is a perspective view of a folded grade board skirt according to the invention with a post, and grade board depicted in phantom and a post sleeve on the post.
FIG. 2 is a perspective view of an unfolded grade board skirt according to the invention.
FIG. 3 is an end elevational view of an unfolded grade board skirt according to the invention.
FIG. 4 is a side elevational view of a grade board skirt according to the invention.
FIG. 5 is a bottom plan view of a grade board skirt according to the invention.
FIG. 6 is a top plan view of a grade board skirt according to the invention; and,
FIG. 7 is a perspective of an unfolded grade board skirt in phantom in the folded disposition and the grade board skirt folded.