The zero article with places

Nouns that refer to certain places and institutions do not take an article when we are talking about the purpose they are used for:

The children are at school. (studying)
What time do you usually go to bed? (to sleep)
The murderer was sentenced to 10 years in prison. (as a punishment)

Other examples are:

church
class
college
court
hospital
market
town
university

But if we mean the place (usually the building) itself, the definite article the is used:

He went to the bed to check if the children were sleeping.
In 1985, there was a fire in the school.
The prison is outside of the city.