Evaluating the Quality of a Wiki

Yes, I believe it is possible to sort bad, good, really good information within Wikipedia.  I think that one entry could more trustworthy over another if the resources were accurate.

If I gave the assignment to make a wiki the assignment would need to be clarified with the expectations explained.  Allowing students to review samples of some bad, good and really good wikis first and ask them to provide reasons they evaluated the wiki to be bad, good or really good.
Discussions in groups would also allow students to begin feel comfortable in discussing their findings. I think following Deters et al, 2010, study in which they provided students with guidelines and a rubric for the assignment would be a great place to start the assignment.

I think students would get confused if you gave them an assignment and then said, "We're going to make a wiki, but Wikipedia is BAD." The constructivist learning style allows students to make discoveries on their own and by saying that Wikipedia is BAD it may taint some students' opinion of a wiki.  Providing students with the opportunity to learn and construct knowledge on their own would give them the opportunity to decide for themselves.  Who knows, perhaps one of the students could come up with a new design for a wiki that would make it better.