Final answer:
The nurse should advise the guardian that the child with varicella can return to school when all the rash vesicles have crusted over, as they are no longer infectious at that point.
Step-by-step explanation:
The appropriate response from the nurse to the guardian whose child has varicella, also known as chickenpox, is that the child should stay home until all vesicles from the rash have crusted over. Once the vesicles have formed crusts, the child is no longer considered infectious and can safely return to school. This is because individuals with chickenpox are infectious from about 2 days before the rash appears until all the lesions have scabbed over.