the is used when we mean School, hospital, university, prison as specific buildings. the is not used when we mean these places as a general meaning.

• Sam’s a student and at school now. (a general idea)

  Sam’s mother is at the school to talk to the teacher. ( a specific building, place)

• He was sent to prison for six months. (a general idea)

• We went to the prison to visit my friend.( a specific building, place)

• Ted stayed at hospital for three weeks. (a general idea)

• He works at the hospital. ( a specific building, place)

• She doesn’t have to go to university. (a general idea)

• Where is the university? ( a specific building, place)

• What time do you go to bed? (a general idea)

• She sat on the bed for a while. ( a specific bed as an object)