I prefer to use Cuttlebone instead as a calicum suppliment....it's actually intended to be ingested by animals (used for birds, and also herps) so I would logically think that is much more safe than a piece of chalk. Especially if it's a piece of chalk, out of the box...how do you know what's in it? Chalkboard chalk is made "mostly" of calcium carbonate, but other types of chalks with a more arts/crafts purpose are made out of plaster of paris. Plus, there could be other ingredients in it that are simply not meant for consumption, because the chalk is not intended to be ingested. Is it non-toxic, yes because it's for people to use. But not producted to people/animals to eat!
Go with cuttlebone...it's cheap, safe, and effective. Just remember to chip off the hard backing before dropping a chunk in the tank. And if your turtles don't like it you still have reptile calcium suppliments like Repti-Cal that you can dust on to their food also, just make sure you read directions and are careful about quantity/frequency.