It depends on the context.
In terms of school, if you mean to say that you are physically inside the school, most people would say "at school." "At" is for describing a location where you are not.
Example: "Where are you?" "I'm AT (the) school."
If you want to say that you are a student studying at a school, you would say, "I'm IN school," as in "I'm enrolled in school."\
"In" can be for location also, but in this case it describes your current status.
I hope this helps! (: