Still, I wouldn’t say it’s illegal, but it’s not really very productive, or professional, and probably not very ethical. Any decisions to be made must be made by the parents or legal guardians, not the grandparents (unless they ARE the legal guardians), so what’s the point of going behind the parents’ back? It’s only going to cause ill feelings, foster a sense of mistrust, and it certainly won’t help the relationship between the school and the parents.

Could you be talking about a situation where a grandparent picks the child up from school and the teacher innocently tells the grandparent something about the child’s day? I wouldn’t call that “going behind the parent’s back” so much as passing along a message at dismissal. (Like, “Sally forgot her lunch today” or “Johnny had a little accident at recess”.)

More details would be helpful…