I would like to ask you when to use "at school" and when "in school". How it differs?
In most cases, we talk about people being "at" school when we're interested in their where they are now:
We use "in" school to refer to the situation or activity:
a: "Where is John at the moment?"
b: "He is at school"
"His son is only 5, but he's in school already,"
Hope this helps