https://forum.wordreference.com/threads/at-in-the-school.37519/
in the school - under the roof of the school building (said of a person OR
thing)
at the school - in or near the school building (said of a person OR thing)
in school - being educated in or at a school; occupied with being educated (
said of a person)
at school - located at a school; attending a class in a school (said of a
person)
You can say that there is an auditorium in the school, but the auditorium is
not in school or at school. You can say that there is a playground at the
school, but the playground is not in school or at school. Auditoriums and
playgrounds cannot be educated or attend classes.
https://forum.wordreference.com/threads/at-in-the-school.37519/page-2