The difference depends partly on what the object of each preposition is. When the object is a place, "at" typically means "to be in the vicinity of a place", while "in" typically means "to be inside of an enclosed place". For example, if I am at the store, I may be inside or outside the building, but if I am in the store, then I must be inside the store. With some types of places, "at" and "in" may have different meanings. For example, if I am in the school, then I am inside the school building, but if I am in school, then I am a student. If I am at the school, I may be inside the school or on the school property outside the building, but if I am at school, then I am attending classes right now.