Alternatively, you can use liquid chalk. This is basically alcohol hand sanitising gel that has chalk powder mixed in. Put it on your hands, wait for the alcohol to evaporate and you should be left with some chalk on your hands. I've heard it's not as good as normal chalk, but I've never tried it so can't really comment.
I found it to be a very good alternative. I would say about 60-70% (totally non scientific estimate) as effective as proper chalk, but much more tidy, esp if you've ever had your chalk pot open in your bag
