Wiki's

No they are not a species in Star Wars.

To find out what a wiki is look at the mother of all wiki sites Wikipedia's entry.

Wikipedia ia probably the most well known Wiki due to its ease of use and the comprehensiveness of its entries.

Is Wikipedia suitable for assignments?
No.
And Yes.

From year 7 up it is common for teachers to ask for assignments to be referenced - this indicates where the student has found the information. When high school students evaluate information it should come from a person or organisation that has authority (expert knowledge) on the subject.

A good reference for an assignment should generally come from a source that is an expert. Governments, Universities and Professional bodies are all examples of expert sources. Books and journals (academic magazines) undergo rigourous examination before being published - this is generally why they are considered good sources for assignments.

Because Wikipedia is a collaborative effort anyone can contribute to entries. While wikipedia monitors entries and tries to maintain their authenticity it may not get to an entry until after your child has used (wrong) information for an assignment.
Wikipedia is a great resource to provide an introduction to a topic - however all information should be double checked and should not be used when referencing assignments (especially at secondary and tertiary level).

As most kids don't have time to double check facts use something you can trust in the first place such as the online version of Enclopedia Brittanica. Or ask a Librarian.