While you could use phrases like "a box of chalk" or "a set of chalk," there's no single word that specifically refers to a collection of chalk like "flock" for birds or "herd" for cattle.
It's possible to create a new collective noun, but it would be a more informal or whimsical usage. You might say something like:
* A cloud of chalk
* A dusting of chalk
* A swirl of chalk
Ultimately, the best way to refer to a collection of chalk depends on the context and the desired effect.