When talking about locations, use at to indicate the general vicinity or area, and in to
indicate inside the building, enclosed area, etc. For example:

at the swimming pool (on site)	in the swimming pool (in the pool itself i.e. in the water)
at the post office/bank (general)	in the post office/bank (inside the building)
at the zoo (visitors—general area)	in the zoo (animals in their cages)
at school	in the classroom

Sample sentences:

I met my wife at the theater. (while watching a movie)

I spilled my drink in the theater (on the floor of the building)

She works at the library on Wednesdays.

She found a rare coin in the library (building).

Dr. Jones works at the hospital every day.

John was in the hospital for a week with a broken leg.

 

For school, prison, and church, the is used to indicate the building. No article indicates
the general situation. Note the following:

"practice"/situation	building
in school (studying, listening to teacher, etc.)	in the school (building)
in jail/prison (staying there as a criminal)	in the jail/prison (temporary)
in church (praying, listening to sermon, etc.)	in the church (building)

Where’s Dad?

in church (attending services)	in the church (fixing the windows)
at church	at the church
in prison (He committed a crime.)	at the prison (visiting his friend)