We don't have confidentiality agreements with the board, however, the school makes any parent who volunteers sign one. This is to protect students' privacy.
As for PTA privacy, the president could easily direct that. For instance ours will say at the beginning of the year that what's said here stays here and that the only way we can be productive and operate as a group is if we can trust that what's said in a board meeting doesn't get blabbed all over town.
Often, when sharing something sensitive, whomever is speaking will say something like 'You didn't hear it from me but . . . " and that's our clue that this info being passed isn't public knowledge yet. (Someone retiring that hasn't announced, etc.)
I'm not sure you have any authority to enforce a confidentiality agreement if someone breaks it. I suspect you'd have to amend your bylaws to say that a board member will be removed if they break an agreement. I mean that's looking at it from a more "legal" statement. I do agree wholeheartedly however, that board members should be informed that they may become privy to information the general membership of the school might not be aware of and they should guard that information as private for the health of the group. I've seen some teachers take parents out of the classroom because they worry that there will be gossip generated about "little billy or susie". It is a priviledge to be that close to what is going on and with that priviledge comes the responsibility of judgement and appropriate behavior. I would hope just discussing it would help. You can of course come up with something written, but you really couldn't enforce it with any authority.
We don't have confidentiality agreements as the behavior that would lead to an infraction is described in our by-laws as prohibited under the Code of Ethics.
Does anyone have their board members sign confidentiality agreements? We recently had an issue with our treasurer and there were some issues on whether or not board members should speak about sensitive subjects with the general membership and the principal.