School

it depends on what u r trying to say:
In school means inside school.
At school we are not sure 100% she is inside school. may be on the road!!!

When someone says i am in school they usually refer to the fact that they are studying in general, they are still attending some kind of school.
When someone says i am at school, it is simply stating that at that momen, they are at school.

I am in school = I am enrolled as a student somewhere
I am in the school = I am currently in a specific building (perhpas reporting a news event for TV)

I am at school = I am currently in the building where I am enrolled (or where I work as a teacher, administrator, etc.)
I am at the school = see news reporter above, here not necessarily inside the building

Hi Qiaohao 
I am at school. (BrE)
I am in school. (AmE) 
See definition below which I've extracted from Collins Cobuild Advanced Learner's English Dictionary. 
If someone is at school or college, or at a particular school or college, they go there regularly to study.
           He was shy and nervous as a boy, and unhappy at school...
           I majored in psychology at H