I think there are some regional variations. In North America: in the school = in the building that is used as a school in school = getting an education at school = on the premises of the school (either in class or elsewhere) Examples: - My favorite spot in the school is the library. - I am 20 years old and still in school because I want to become an engineer. - What did you learn at school today? I got in a fight at school this morning.

Correction: What is the difference between saying in school and at school? He is in the school or he is at school? Is there any difference? Thanks for your answers He is in the school = he is inside the school building His at school= he is somewhere around the school, maybe in the playground