تا اونجایی که من فهمیدم:
at school means the person literally,physically inside the school
طرف بصورت فیزیکی تو مدرسه است
where is your brother? He is at school
in school means the person is in studying in general but not necessarily inside the school buliding at the moment
بصورت کلی یعنی طرف در حال تحصیل هستش ولی لزومی نداره که توی اون لحظه ای که استفاده میشه طرف بصورت فیزیکی واقعا توی مدرسه باشه.
My husband doesn't have a job because he is in school.he is studying law.
