Polygamist families...?

If a teacher sent home a note with a child to be signed by their parent and the child was from a polygamist family where his father had several wives, who should the signer of the note be? Would it be appropriate for the child to go to one of the other wives (not his biological mother) and have her sign the note or is it required that one of his biological parents signs the note? Do polygamist families have certain guidelines in place for such instances? How should a teacher respond if the note was signed by one of the other wives?