I'm 18, my foot is kinda flat but not completely, i use the DR.Romanov style running but it kills the heck out of my shins, dang!
At your age, you shouldn't have a problem running the 1.5 mile. Not to be an ass but if you are going to boot camp, which means you're physically qualified, there's no excuse for having issues running. A 12:30 is for us older types who have had broken bones, blown out discs (same disc twice and two others in my case), knee surgery, etc. I tell ya, it just dissapoints me when I see sailors come out of boot camp looking physically inept.