Do you have one of those little “scrubbies” made by crocheting tulle? My MIL used to make them and they work great for getting chalk off.
While im thinking about something pertaining to marking i will pass this along... I recently saw where a quilter on the internet dumped out the chalk powder in her refillable chalk marker (the one with the little wheel to help with marking fine lines). Then she refilled it with some of the new Ultimate Pounce powder that can be removed by touching it with an iron. I thought it was a great idea, although the only color currently available in iron-off is the white. I have no idea if other iron off colors are in the works at this time. Pounce currently does also have a pencil (actually more like a crayon that you need to sharpen) in the iron off form - again only in white.
Thought i would pass that along!