Can a University put a Creative Commons document containing a NonCommercial element (NC) behind a wall?
Creative Commons "NonCommercial" prohibits uses that are "primarily intended for or directed toward commercial advantage or monetary compensation."  Under U.S. law, where exactly the boundary lies can only ultimately be determined by development of precedent in case law, since the U.S. has a common-law legal system.

It is pretty clear, though, that this type of use would be non-infringing, for the following reasons:


Virtually no person pays to become a university student for the primary reason of access to online documents.  Likewise, it is an exceedingly minor perk for employees and could not be considered to give the university a significant commercial advantage (rare historical documents might be a different matter).
The actual barrier is a University ID, which can typically also be acquired in many ways that do not involve compensation passing in either direction (e.g. research affiliates).