Here's a somewhat simplified take which is not a hundred percent correct but is easy to remember and mostly true. * at school - somewhere in the premise of the school Use this to mean someone who regularly goes to the school(students, teachers) is physically there. * at the school, at Tommy's school, at her son's school, etc. If a person is at the school for some special reason, you say it with "the" or another modifier for "school". * In school - means "in the school education system" For example, you might say "We learn American history in school", or "I learned all that in school".

I believe this is one of the peculiarities of the English language that native speakers don't really pay that much attention to anyway. For this instance I guess it's a complicated comparison between enclosed and open spaces. 'She is IN school' can mean: she is IN the school building (enclosed within walls), such as the classroom, the cafeteria, the gym, etc. It could also mean something less physical and more figurative: she studied Mathematics in college (the 'college' here refers to a college education rather than a college building). 'She is AT school' could possibly be more general and would include open spaces as well, such as the school garden or basketball court. 'She is AT the classroom' also works here. 'AT' school, in my opinion, refers to a more physical context. English prepositions sure are annoying... BUT, having said this I don't think this in/at comparison makes much of a difference. To me they are interchangeable, so don't worry too much about it.