Hi Phillip,

I understand I won't ever be able to prevent students from opening up
another spreadsheet and manually copying the formulas. That's fine.
Note: I suppose I could have not only unique identifiers but also
unique assignments. That would make it pretty hard to create and
grade, though, unless the assignments only varied trivially from
student to student, so I'd rather not go with unique assignments.

You might share the workbooks and turn on track changes.
That will make Excel record each edit and store it with the username that did
the edit. But if student A works on his assignment on Student B's system, that
won't be of much help, unless he also works on his own system.

Still, having two usernames in the change list is no proof of fraude.
The username recorded is the one in tools, options, general, which can simply
be changed.

Regards,

Jan Karel Pieterse
Excel MVP
http://www.jkp-ads.com
Member of:
Professional Office Developer Association
www.proofficedev.com