the 2 (school / the school etc.)

Compare school and the school:

Ellie is ten years old. Every day she goes to school. She's at school now. School starts at 9 and finishes at 3.

We say a child goes to school or is at school (as a student). We are not thinking of a specific school. We are thinking of school as a general idea – children learning in a classroom.

Today Ellie's mother wants to speak to her daughter's teacher. So she has gone to the school to see her. She's at the school now.

Ellie's mother is not a student. She is not 'at school', she doesn't 'go to school'. If she wants to see Ellie's teacher, she goes to the school (= Ellie's school, a specific building).

We use prison (or jail), hospital, university, college and church in a similar way. We do not use the when we are thinking of the general idea of these places and what they are used for.

Compare:

When I leave school, I plan to go to university / go to college. (as a student)

Joe had an accident last week. He was taken to hospital. He's still in hospital now. (as a patient)

Ken's brother is in prison for robbery. (He is a prisoner. We are not thinking of a specific prison.)

Sarah's father goes to church every Sunday. (for a religious service)

I went to the university to meet Professor Thomas. (as a visitor, not as a student)

Jane has gone to the hospital to visit Joe. She's at the hospital now. (as a visitor, not as a patient)

Ken went to the prison to visit his brother. (He went as a visitor, not as a prisoner.)

Some workmen went to the church to repair the roof. (a specific building)

With most other places, you need the. For example, the station, the cinema (see Units 72C and 73D).

We say go to bed / be in bed etc. (not the bed):

but

go to work / be at work / start work / finish work etc. (not the work):

go home / come home / arrive home / get home / be (at) home / do something at home etc. :

→ The units 72–73, 75–78 · Prepositions (at school / in hospital etc.) units 123–125 · Home unit 126C · American English appendix 7