The dolphin is wise. These statements are about all dolphins, a dolphin in general, not specific dolphin or group of dolphins. We may use some nouns both with and without the . We use the nouns without the when we are talking about the normal purpose of an institution rather than about a specific building. Otherwise we use an article with them.
School starts at nine o'clock.
The school is in the center of the village. The guilty men were sent to prison. Vegetables are delivered to the prison twice a week. There are a number of other nouns which are without the in similar contexts. I am usually in bed by eleven.