Force Yosemite to forget wireless credentials?

Question

I manage a few dozen MacBooks for a university, which are shared among faculty. They are checked out for a maximum of a week at a time, and some are delivered on a per-class basis as well.
I need to be able to force Yosemite to forget the credentials they use to sign in to our wireless network every time they disconnect, turn off the wireless radio, or shut the computer down. If the credentials are saved, this creates a situation where one user may turn on the computer, be connected automatically under a previous users credentials, and then download something illicit or worse, introduce a virus to the network.
I have tried leaving the "remember networks" box unchecked, but for whatever reason this does not prevent OSX from creating a keychain store for the first user name and password that is used to sign in to the network. Is there any way to make sure that OSX asks for a user name and password every time someone connects to our wireless network?

Answer 1

Thanks Slyder.
That seems to have worked. When the ATV started up again, the preferred network was selected. Thanks.
What I didn't think of until later was this:
The Remote HD application (iOS client, ATV host) enables you to run applications on the ATV, including System Preferences. It also provides a trackpad and keyboard interface for the ATV. I ran Network Preferences and deleted the unwanted network. Now I'm sure it will start with the right one!
BTW - Remote HD <http://www.remotehd.com/AppleTV/Overview> is a great app for remote access to your Mac and your ATV.