Some students will just watch during your demo. Others will duplicate your steps on their computer as you demonstrate. Therefore, the demo should leave the data that will be used in the upcoming exercises totally untouched. That is, data changed by the demo should not affect the data used in any of the exercises.

You may be tempted to accomplish this by using a different database or files for the demos and exercises. That would definitely segregate your demo and exercise data. However, it also requires the instructor to switch between two different databases/files. This may present a problem for the students who want to follow the demo on their own machines. Before embracing this approach, you must ensure that the students can easily switch databases/files.