Hi and welcome!
Although a daunting task, I agree with your principal that this is a move in a positive direction. I am in a school where we also have no set " handbook" for volunteers, although our board, and the principal and other admins' are very close, and have been for the 9 + years I have been at the school.
Although I have always wanted to create better guidelines, time has always prevented me from going that extra 10 miles to do so. The few " job descriptions" I have done, have come out of difficult situations with volunteers, and I have done so out of necessity.
I think by having clear-cut guidelines, it may help reduce some of the in-fighting, and gray area. It will also put all the teachers on a more level playing field, because each will receive comparable volunteer assistance.
Having said that, the REALITY is that some volunteers have more time to give, some teachers LIKE having more help than others, and of course matching personalities, and how those relationships evolve, are all things you CAN'T control.
Most importantly, it is a VOLUNTEER position in a public school, and youare at the mercy of the people who chose to help. It is not something like Junior League where you pay dues for the privilege of being assigned X # of hours of community service a year.
If you make your " guidelines" too stringent, I think you will scare people away. I would tend to make them ALL simple and brief, thereby satisfying the request of the Principal. That also leaves room for a little creativity.
I have also said somewhere in another thread that we have changed our language to " volunteer opportunity".. rather than " WE ( the board) need people to help US"... that has changed the focus to the kids ( which is why we are all there).... as opposed to people resenting us and not wanted to help " those cliquey women". We have had AMAZING positive results with this, in just a few months.
So, each of your pages can say " Volunteer Opportunity for Room Parent, or Field Day, or whatever you do.