Unauthorized uninvited admin on site

This is the first I've heard of any cases like this. My guess - you may have either:

1. Inadvertently invited users with the wiki Administrator invite key
2. Chosen an easily-guessed administrator invite key

To resolve this problem ASAP, please do this:

* While logged into your wiki, click on Settings (near the top right)
* click on "Keys and Access" - you'll notice on the right side, there is a list of user levels (admin, moderator, etc).
* Near the right side of the screen, you'll see a link that allows you to change the wiki administrator invite key - change this ASAP.
* Immediately remove the user involved (or change his/her access controls to the proper level)

By changing the wiki administrator invite key, that will disallow anyone else from adding themselves as a wiki administrator.

You may also want to set keys for the other access levels (moderator, contributor, reader) and invite students with the invite key of the access level you'd like them to have.

I also notice you put in a support request about this, I'll reply to that in just a moment as well.