Unfortunately subdomains are not offered by USD at this time except as redirects.
A subdomain is the following: http://law.sandiego.edu > law is the subdomain for sandiego.edu.
That is, the address law.sandiego.edu redirects to sandiego.edu/law. The subdomain URL can function as a redirect to the main www server but cannot serve as the primary URL for any department, school, or center's website at the University of San Diego.
Moving from the current architecture (sandiego.edu/law) to utilizing a subdomain (law.sandiego.edu) can create some very significant search engine optimization issues, where there is a high potential to lose more than half of your ranking.