Talk to the principal instead. They (staff) are bound to confidences, volunteers are not. They may also have more advice to offer due to experinece and training.
I do not see anything wrong with consulting with another board member as long as she understands the confidentiality of the matter. We had a situation with a board member last year that I discovered. My feeling was that I needed to share this info with another board member not only to get their take on it, but to protect myself from looking like I was covering something up if someone else discovered the problem.
I am having some issues with one of my board members in that she continually volunteers for work and later offers an excuse as to why the work was never done. Now it has come to my attention that she has lied to me on a fairly important matter, which leaves me wondering what else she may have lied about.
As I am pondering how to handle this, I am wondering if it would be inappropriate for me to discuss this confidentially with another board member whom I respect and trust in order to get her take on things? I don't want to gossip, or for this to be construed as gossip, but I would feel more confident in my decision on handling this if I knew the other board member has noticed the same pattern of behavior that I have (this other board member is not aware of the lie).