Why isn't there any article in the expression "vary from school to school"?

I am a non-native English learner. An example sentence in LONGMAN dictionary confused me. In the sentence "Test scores vary from school to school", there is no article in front of either school. I would expect that an a would be in front of each countable word school.

Is there some grammar rule that can explain this sentence?

Solution 1:

There are a number of constructions involving two noun phrases that, when used in a general sense rather than referring to particular items, omit the article. The two items will always be in some sort of close relationship, but the meaning can be very varied.

Examples are:

X and Y: "husband and wife"; "rod and gun" (i.e. fishing and shooting); "bat and ball".

from X to Y (especially from X to X) "from school to school"; "from hand to mouth"; "[a grin] from ear to ear".

X after X: "time after time"; "person after person"; "game after game".