I never understood why a principal,or any boss for that matter would want to be Facebook friends with someone who worked for them (or why anyone would want to be friends with their boss either) but my daughter set me straight on the former.
When two people are Facebook friends they have access to each others personal data. The principal or boss can keep a blank page giving no one access to their personal information and at the same time learn who the employee hangs out with, what he does on his free time, and what she is interested in. Many have gotten in serious trouble because of this page.
As for a teacher friending a principal? What for? Brownie points? The friendship won't get you anywhere. My advice--DON'T DO IT!