He works in a school. (He's a teacher). He's at school. (He's a pupil or student). He's at work (teacher).

to say someone is currently located within the school, we would say “at school”: -I am at school (I am located currently at school) -I am working at school (I am getting some work done at school) -I’ll meet you at school Also if you are a teacher or your job is at a school you say: -I am a teacher at the school. -I work as a janitor at the school. To say you are “in school” is usually to say you are enrolled in school for a subject or period or time/are currently a student: -I am in school for the spring* -I am in school for nursing -I am in school all year *We also say “out of school” if you have graduated or on break from the semester. -I am out if school for the summer.

In school has another meaning in the US. In school can mean that you haven't finished your education yet. Someone might ask someone if you have started a job. If you haven't finished your education you would answer, "No." I am still in school (even though you are not physically in a school.)