No one truly knows how dust resistant the watch is because Apple has not obtained an official rating for it. The Apple Watch is rated IPX8 with the X signifying that it does not have an official rating for dust resistance while the 8 designates the level of water resistance. The only thing that Apple has really mentioned is to not expose the watch to large amounts of dust, which I'm certain would include the chalk.
To be on the safe side, I would not use it anywhere around the chalk as I'm sure they can get into the holes.