I have a class using WordPress.org rather than .com this term. In the past (using .com) I've simply had them start a new .com site called "yourname_practice" so they could mess around and not destroy the work they'd already done on their main site.
On .org I'm not quite sure how to do this. They have created their sites on their purchased domains. I'd like to have them change things within the CSS but again, not destroy what they've done. It might be that I have them save and then restore? Or is there a practice plugin that would work?