The word "school" is used as a countable and an uncountable noun.
It's an uncountable noun used without an article when you are talking about somebody attending a school. It's a countable noun if you are talking about somebody doing something other than learning or studying there.
She's still in school = She is still studying there.
If you say "She's still in the school", it doesn't mean she studies there; instead, she is there for any other purpose e.g. just visiting or paying your tuition fee.
She's going to school. It means she's a student there.
She's going to a school. Maybe to find a job or to see how a school looks like.