Can I make a wiki that my students can read but not edit?
- Although you used to be able to do this in older versions of Moodle, it's not really in the "spirit" of a wiki and is no longer possible without tweaking permissions. A Book might do the job just as well, or a simplePage.
- However, if you really need to do this then remove the student role from mod/wiki:editpage and mod/wiki:createpage when you create your wiki. See forum discussion here: 
Students can't see each others wikis
If you have set the wiki to "individual", then only you the teacher and each individual student can see them. If you would like them to collaborate on a wiki, choose the collaborative option. However, this will just give one wiki for all the students.
How can I grade my students' wikis?
There is no in-built grading facility in the wiki but you can manually add a grade item in the Gradebook called (for example) "yourwikiname" and enter your grades there.
Can I export a wiki?
While it was possible in earlier versions of Moodle to export wikis, for instance as zipped files, this is not the case in 2+. There is a tracker entry here MDL-24439