Grandparents have NO legal rights in California. regardless of the relationship (unless it was dad) you can't sign a permission slip for a child.She is a volunteer. SHE IS A VOLUNTEER!!! She can't sign a permission slip.
Yes, IMO, it is. Based on what you posted on this forum. This is the childs GM. Not a random stranger. And Dad KNEW about it. It really is not that big of a deal. But if you feel so put out that your little one went on a flippin' field trip...have at it. I will say this is going to be a long and PAINFUL ride for your child/ren.