Umm, wasn't something like this a standard thing already long ago? I gave the exercise lessons for programming courses at TKK (now Aalto University) over 25 years ago, and in the bigger lecture halls, we had a system where there were four or five blackboards rotating up/down with an electric control. So you'd fill one blackboard, and then push a button, and the used up blackboard would lift up, remain visible, and you'd go on with the next blackboard. The system looked like it had been in use since the 1960's when the campus was built.