Final answer:
A parent demonstrates correct understanding of when their child can return to school after chickenpox by stating the child can go back once the lesions are crusted. Chickenpox, caused by the varicella-zoster virus, is infectious until lesions have scabbed.
Step-by-step explanation:
The nurse determines the teaching is effective if the parent makes the statement: "My child can return to school when the lesions are crusted." This indicates that the parent understands that children with chickenpox can return to school once all the chickenpox lesions have formed a crusty scab and are no longer infectious. Chickenpox is a common childhood disease caused by the varicella-zoster virus, and individuals are infectious from about 2 days before the outbreak of the rash until all the lesions have scabbed over.