I haven't gone to a school where I didn't know anyone since I was 9, so my experience won't be too relevant.
However, when I started at my school's sixth form there were several people who came in from different schools, and every single one of them has made good friends with the rest of the sixth form. None of them had mentors or anything, but people will always say hello and talk to new people, so making friends shouldn't be too hard.