In some common fixed expressions of place, time and movement countable nouns are normally treated as uncountables, without articles. Here are the most common expressions of this kind:
To school / at school / in school / from school
To university / at university / from university (GB)
To university / in university / from university (US)
To / at / in / into / from church
To / in / into / out of bed / prison
To / at sea
To / in / from town
At / from home
Leave home / school / university / college
Start school / university / college
Enter school / university / college
To / in / into / out of hospital
To / at / from work
By car / by bus / by bicycle / by plane / by train / by boat
On foot
By radio / phone / letter / mail
In some cases, place nouns are used with articles. There is usually a difference of meaning. For example, when we say at the university, we are referring to the building, not to the activities going on there.
Compare:
John has to go to hospital. He has got heart problems. (as a patient.)
I went to the hospital to see James. (Here we are referring to the bricks and mortar structure.)
Notes
In American English, the words hospital and I are always used with articles.
