Huzzah I managed to make a new sandbox, and can now teach my students how to do it. Graeme Bartlett [1] all the way on this one (the following is a modified version of his instructions):
To make a sandbox, type the name into the search box. For example User:Aagwebb/WikiTeachingSandboxNumba2
Then press search. It will say there is no article, but gives you the option to start it. Click that "start" link. And then input some preliminary content and click save.
You can have many pages listed with your user as the start of the name. This is counted as user space. The default name is "sandbox" but it can be any other name.