How to prevent calluses from writing on chalkboard?
A couple ways. You may consider products called "chalk clip" or "chalk holder" like this one:



They make the girth larger and less likely to produce a small pressure point on your finger, which causes callus.

Another option is to get some "foam tubing" from general hardware stores. They are cheap and in different sizes, thicknesses, and even colors. Cut a 2 to 2.5 inches long segment and put it over your middle finger. To improve comfort when bending your finger you can make a small vertical incision on the tube (palm side) so that you can bend the finger more freely.

Volleyball players also have elastic finger guards like these:



You can get a set of them from sports stores. Just pull it down a bit to protect your last joint rather than the middle joint. My experience (as a volleyball player not chalk writer) is that the ones with nylon outside and a thin foam layer inside work are the most comfortable.

If technology permits, you can also consider projecting your hand writing using a projector, computer tablet, or even interactive screen. Those methods allow you to write with a lighter grip.