Notwithstanding any provision of any law, no school district, officer of any school district, school principal, physician, or hospital treating any child enrolled in any school in any district shall be held liable for the reasonable treatment of a child without the consent of a parent or guardian of the child when the child is ill or injured during regular school hours, requires reasonable medical treatment, and the parent or guardian cannot be reached, unless the parent or guardian has previously filed with the school district a written objection to any medical treatment other than first aid.