Why?
That's an excellent question for someone at the school and a poor question for anonymous strangers on the internet who know nothing about the policies of some unknown school in Pennsylvania (it's also a question you should ask your child's medical providers). That said, why can't you register your son yourself?
A power of attorney ("POA") is a document by which one person (the principal) confers on another person (the agent or attorney-in-fact) the authority to conduct transactions on the principal's behalf with third persons. A POA can be very broad or very narrow in scope.
A guardianship (whether temporary or permanent) is a proceeding whereby a court gives a person other than the child's parents authority over the child's person and/or estate (i.e., assets). A guardianship does not terminate the parent's rights but suspends them as long as the guardianship exists.