You can establish residency during school. People usually keep their former permanent address during college though.
http://www.scsc.pa.gov/Job-Seekers/Pages/PA-Residency-Information.aspx
"As a current student at a college, university, or technical school or as a member of the armed forces, you meet the requirements of Pennsylvania residency if you...are a student from another state who is currently residing at and enrolled in a Pennsylvania college, university, or technical school, and have a Pennsylvania mailing address."
I was mainly addressing the part of that comment that addressed the number of med students who are Pitt alums, not necessarily the IS/OOS ratio.