I claim the ornamental design for a stand for writing boards, as shown and described.
FIG. 1 is a perspective view of a stand for writing boards showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a perspective view thereof shown in use.
The broken line showing of two small writing boards, such as blackboards, in FIG. 8 illustrates the environment of the claimed design and forms no part thereof.