Articles are not normally used in some fixed expressions of place, time and movement.

Examples are:

To school / at school / in school / from school
To university / at university / from university
To / at / in / into / from church
To / in / into / out of bed / prison
To / in / into / out of hospital
To / at / from work
To / in / from town
At / from home
Leave home
By day / at night
By car / bus / bicycle / plane / train / boat

Leave / start / enter school / college / university

Notes

Similar expressions with articles often have different meanings.

I met her at university. (when we were students)
I will meet you at the university. (Here we are simply referring to the building / place.)
Susie is in hospital. (She is undergoing treatment.)
I met an old friend at the hospital when I was visiting Susie. (Here the hospital is just a meeting place.)

In American English, university and hospital are not used without articles.

Articles are often dropped in double expressions, especially when they are used with prepositions.

Husband and wife (More natural than ‘the husband and the wife)
With knife and fork (NOT with the knife and the fork)
From top to bottom (NOT from the top to the bottom)

After both and all

We often leave out the articles after both and all.

Both girls sing well. (OR Both the girls sing well.)
All three brothers were punished. (OR All the three brothers were punished.)

After kind of, sort of, type of etc

Articles are usually left out after kind of, sort of, type of and similar expressions.

What kind of person is she? (More natural than ‘What kind of a person is she?’)