When should exceptions to established rules be granted?

People often oppose some proposed exception to the rules on the grounds that doing so would set a dangerous precedent and engender abuse. For example, suppose that an honest and diligent student is in the hospital, and he wants to keep up with his school work as much as possible. His parents propose that he take his math exam from the hospital, and they'll monitor him during the exam. The school refuses on the grounds that if all students were allowed to do that, then cheating would be rampant because not all parents would be honest or diligent monitors. Is that a valid reason for refusing this proposed exception to the rules? When should exceptions be granted to established rules?