Professor uses Wacom and Inkscape to draw a picture, which is incrementally transmitted to students' computers. Students, those who have Wacom, may interact. Or just watch. Transmission happens every time the svg file is saved. Transmission requires a RabbitMQ server, which can be easily set up. Basically, a class needs one person who knows Linux, to set up the server.

It is intended for scientific collaboration or teaching in small groups of people. I am now using it for teaching my QFT class, although it only has 5 students. In principle, it should scale, but I have not tried it for large groups...