Hello Warren, "He finished school at 15". Here the past simple tense is used to indicate an action that terminated fully in the past. Someone tells about this person's past,including the fact that he finished school at the age of 15. "He had finished school at 15." You wouldn't use the past perfect tense unless this action of finishing school in the past is prior to another action in the past as well. "He traveled to Paris after he had finished school at the age of 15." Here someone is telling about past events,however the action of finishing school is prior (comes before) the action of traveling to Paris. In this case the earlier action in the past uses the Past Perfect tense and the following action the Past simple tense.

Hi the first sentence is correct.