[susan53]

Compare these examples and think about the question Why is the person /thing in the place referred to?

A : Where's John? B : He's in hospital vs He's at the hospital.
A : A : Where's John? B : He's in prison vs He's at the prison.
A : Where's John? B : He's at church vs He's at the church.
A : Where's John ? B : He's at school vs He's at the school
A : Where's John? B : He's in bed
vs - A mother and young child are playing "Guess where I've hidden Teddy" - Mother : Is he in the bed?

You should have noticed that the meaning changes :

He's in hospital = he's been admitted as a patient, he's being treated for an illness
He's in prison = he's serving a prison sentence
He's at church = he's worshipping
He's in bed = He's asleep or resting
He's at school = he's studying, he's a pupil (or possibly, he's teaching, he's a teacher).

In other words the article is omitted when the real meaning of the sentence is to tell you what the person is doing in that place, which is always the "usual" thing which you would expect a person to be doing there.

Now compare the sentences with the article, and you'll see that they only tell you where the person is, without explaining why - but it isn't for the reason that would be understood without the article.

He's at the hospital = Perhaps he's a doctor, or nurse, or cleaner... Perhaps he's having a check-up, or visiting someone
He's at the prison = Perhaps he's visiting someone, perhaps he's a warder, or a cleaner, or a social worker ...
He's at the church = perhaps he's a priest, or a cleaner, or is taking brass rubbings, or arranging the flowers ...
He's at the school = perhaps he's the caretaker, perhaps he has gone to a parent's meeting ...
He's in the bed = that's where I hid him.

With these sentences, the reason why can only be understood from the context. For example, if I know that John is active in the PTA and you tell me He's at the school, then I may asume that the reason is a PTA meaning. But that's inherent in the context. The sentence itself only tells me where he is.

With some places there is no "usual" activity - eg you can go to the park to walk your dog, take the children to play, have a picnic etc etc. So therefore "the" is always used.

And it's exactly the same with the verb go or any other verb:

I'm going to school ( I'm a pupil/teacher and I'm going in order to teach/study - the "usual" activity)
I'm going to the school (I might be a cleaner, or a parent, or a social worker... You know where I'm going, but only the context tells you why)