The right to withdraw a child from school

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Recently I had a problem: my grandmother was not allowed to take my son without notary trust. The school administration said that the person who will take and take my child out of school should be trusted. What if I have a few such persons - a grandmother of the child, his grandfather and my brother? Should there be a few trusts, or is one enough? Just before the children without confidence were given, because all of my “chargers” are known in the face. And how to write this trust: by hand and then to sign?