How likely am I to see someone from sixth form at the same uni as me?

I know it depends on how big the uni is and how often the person walks around and such but just generally... there's someone I know from my sixth form who I think might be going to the same university I'm going to but we're doing completely different courses so I know I won't see them in lectures and our timetables could be different but is there a chance that we could go the whole 3 years without bumping into each other or is that highly unlikely? Not that it's a big deal if I did bump into them but I'd just rather not. For personal reasons