Questions like this one are almost always determined by a group's own bylaws -- if the bylaws don't exclude the principal or other specific people (teachers, for example) from holding a board position, then it's allowed. There aren't any "master bylaws" for PTOs or other independent parent groups -- each individual group decides its own bylaws, policies, and governing rules (although there are certainly some common practices shared by many different groups.) If your group doesn't have bylaws at all, check out the samples in the Bylaws/Policies section of the File Exchange.
- lharac
Links in this post:
Bylaws/Policies section of the File Exchange: https:/classic.ptotoday.com//filesharing/category/62-bylawspolicies