A. Compare school and the school:

#1 Alison is ten years old. Every day she goes to school. She's at school now.
School begins at 9 and finishes at 3.
We say a child goes to school or is at school (as a pupil). We are not necessarily
thinking of a particular school. We are thinking of school as a general idea.

#2 Today Alison'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.
Alison's mother is not a pupil. She is not 'at school', she doesn't 'go to school'.
But if she wants to see Alison's teacher, she goes to the school (= Alison's school, a particular school).

B. We use prison, hospital, university, 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:

#1 * Ken's brother is in prison for robbery. (He is a prisoner. We are not thinking of
a particular prison.)
* Jack had an accident last week. He was taken to hospital. He's still in hospital
now. (as a patient)
* When I leave school, I want to go to university.
* Mrs Kelly goes to church every Sunday. (to a religious service)

#2 * Ken went to the prison to visit his brother. (He went as a visitor, not as a
prisoner.)
* Jill has gone to the hospital to visit Jack.
* She's at the hospital now. (as a visitor)
* Excuse me, where is the university, please? (= the university buildings)
* The workmen went to the church to repair the roof. (not for a religious service)

With most other places, you need THE.
For example, the cinema, the bank, the station.

C. Bed work home
We say: 'go to bed/be in bed' etc. (not 'the bed'):

* It's time to go to bed now.
* This morning I had breakfast in bed.
but * I sat down on the bed. (a particular piece of furniture)

'go to work/be at work/start work/finish work' etc. (not 'the work'):
* Ann didn't go to work yesterday.
* What time do you usually finish work?

'go home/come home/arrive home/be at home' etc.:
* It's late. Let's go home.
* Will you be at home tomorrow afternoon?

D. We say 'go to sea/be at sea' (without 'the') when the meaning is 'go/be on a
voyage':
* Keith is a seaman. He spends most of his life at sea.
but * I'd like to live near the sea.
* It can be dangerous to swim in the sea.