@ESLgirl,
The students (in / of) my class are noisy.
Both okay, 'in' more common.
The teachers (in / of) ABC School are kind.
First choice 'at'; other two would work.
The people (in / of) in Japan were devastated by the earthquake.
Both work.
The staff (in / of) Starbucks are friendly.
First choice 'at'. Other two would work.
If there was greater context, it would probably help.