Tohle jsem našel na netu:
Hmm… I think we're missing something here.
At school, to me, has a concrete meaning. Located at the school you attend in
a physical sense. Example: I have been at school since 7 AM.
In school, on the other hand, has a conceptual, more abstract meaning.
I will be in school until I graduate. I have been in school for
8 years now.
So, if I was 18, and my mother asked me “how many friends do you have at
school” I might answer with the number who attend with me, at the same
school.
If she asked me “how many friends do you have in school” I might also
include additonal friends who do not attend my school, but who are in school
somewhere, rather than drop-outs or in the work force.
So I think both have particular meanings, at least here in the Great Lakes
region.