If it is not an oil - based chalk, you should be able to remove it with brushing and washing - before you wash or iron it. Dritz fabric stain remover can help and fabric erasers or artist putty erasers are good as well. You dab with a putty eraser, and be careful that it does not have a lot of oil in it. Failing that, using Fuller's Earth can be good. All of these things should be done before you 'set' it with washing and ironing.