How would you ask if someone was "in school" vs at school. Like when asking if someone is going to college by asking "are you in school/are you going to school"?
There is no distinction between the two. Each place is use with only one presposition when you are "at" it.
If someone attends a school, a colledge, or a university, you say they "go" there (for school) or "study" there (Он ходит в школу / Она учится в университете)
Thank you. British English also uses "at school" to refer to both the temporary fact (where you are today) and whether you are enrolled.