I've put together the very key I used as an apprentice.  It looks completely functional as far as keys go.  In regards to opening Karazhan, however, this might as well be a piece of scrap metal.$B$BWithout Medivh's consent, the key is worthless.  Yes, of course I know Medivh is dead.  This merely makes things difficult, not impossible$B$BThere is a place in Azeroth called the Caverns of Time, guarded by the bronze dragonflight.  Find a way inside, travel back in time and get Medivh to enable the key.