In some fixed expressions, countable nouns are used without articles.
To school / at school / in school (US) / from school
I am going to school. (NOT I am going to the school.)
We were friends at school.
Jane hasn’t returned from school.
To / at / from university / college (GB)
To / in / from college (US)
I first met her at college.
To / at / in / into / from church
To / in / into / out of bed / prison
Mom has gone to church.
To / in / into / out of hospital (GB)
To / at / from work
To / at sea
To / in / from town
At / from home
Leave / start / enter school / university / college
By day; at night
With place nouns, similar expressions with articles may have different meanings.
I met her at university. (when we were students)
I will meet you at the university. (In this case, the university is merely a meeting place. Here the reference is to the building.)
Susie is in hospital: she has kidney trouble.
I went to the hospital to see Susie. (Here the reference is to the hospital building.)
In American English, the nouns hospital and university are not used without articles.
We were friends at university. (GB)
We were friends at the university. (US)