Math via Skype on a Mac?
Did you try the screen share?

It does require you typing, but not in the Skype field.

You could use excel or word to type, and then share your screen with others.

More details HOW to:

https://support.skype.com/en/faq/FA10022/how-do-i-share-my-screen-in-skype-for-mac-os-x

You will also need a digital sketch pad like the :

http://www.wacom.com/en/products/pen-tablets/bamboo/bamboo-connect#tabpane2