work in / at

The trouble I am having in explaining this is that, loosely speaking, both are said. I am looking for a “rule” to make this easy — and I can’t find one.

The best that I can come up with is: use ‘in’ for a specific department/area and ‘at’ for the general place.

Ex. She works in the German department at Village High School.
Joe is a custodian at Village High School. He works in the school in the Electrical Division/Department.
Mike is also a custodian at Village High School. He works outside and his job is to keep the grounds clean.

I am afraid I didn’t explain this very well. Wait for more responses…