I once stopped by my old grade school to visit a teacher or two. I didn't call first, just stopped at the office first to check in, and gave specific classroooms and teachers I wanted to visit.
This was back in the 70s, though, before everybody in schools was (with good reason!) scared of everything.
You might call your headmaster and see what the protocol is now. If there are some teachers you'd like to re-connect with, maybe you could message them on Facebook and ask them if they're available on a certain day.