Final answer:
The nursing diagnosis that fits the situation is 'impaired social interaction' due to the child's ongoing oppositional behavior, which may signify an underlying condition such as ADHD or autism spectrum disorder.
Step-by-step explanation:
The most appropriate nursing diagnosis for a school-age child demonstrating ongoing oppositional behavior at home and at school would be impaired social interaction related to short attention span and distractibility.
This diagnosis considers the possibility of underlying conditions such as attention deficit hyperactivity disorder (ADHD) or autism spectrum disorder, both of which can manifest as difficulty in social environments and can cause challenges with following instructions, thus impacting family dynamics. The diagnosis acknowledges the child's struggle with social cues as well as the subsequent interrupted family processes stemming from the child's behavior.