We ended up not incorporating, but when we WERE working on this, we always planned to use the principal. Our principal is a member of the PTO Board and the most longterm candidate.
Did you use a local attorney to originally help you incorporate? Perhaps he/she would be willing to act in that capacity, especially if your group still ocasionally consults with the attorney. Or if you have a parent or former parent who is an attorney with a local law firm, they might agree to do it.
Another alternative is to pay a company to act as your registered agent. It's been a number of years since I did that, so I don't know how expensive it is. It used to be $25/year or so; it's probably way more than that now, but might be worth checking into.
I actually would have thought your principal would have been the perfect person, especially since you probably use the school's mailing address as your own.
My term expires at the end of the year. I am concerned as to who to put as the Statutory agent, I don't feel comfortable putting the principal on it. Does anyone have any suggestions?