could you use clay to make a 'piece of chalk' and embed a magnet in it? I have a kit from hobbycraft containing clay and magnets that cost about £6 and im sure you could find cheaper! then once youve made the chalk, glue/tape a little real chalk on the end of it? Make the chalk piece a little larger than normal so it doesnt break! from afar no-one will no the difference? I dont know if thos would work but you could always give it a go!