rashinI work in a school. I work at pidgin school.
Hi! Rashin. You can say "in" and "for" If you want to say you work inside the school or you are one of the worker of the school.Both are correct and can be interchanged in your case. You can say "at" If you work near the school or somewhere around it but this doesn't imply you work in the school.
These expressions you are asking about are quite idiomatic. What you say with the verb 'work' may not be the same as what you say with the verb 'study'.
In the examples you have provided above, "in" and "at" do not mean the same thing. However, they are frequently interchanged. Strictly speaking, if you work in a school, it means you work inside of the school itself. You might be a teacher, a janitor or a secretary in the office, but by saying "in" you indicate that you work inside of the building. If you work "