The phrases "in school" and "at school" are both grammatically correct, but they have slightly different meanings.
In school
"In school" is used to indicate that someone is engaged in the educational process as a student in American English.
My daughter is still in school. (= My daughter is still a student.)
I spent a lot of time in school studying for my exams.
I was a shy kid in school, but now I’m much more outgoing.
"In school" is also used to indicate the time during the day when children spend in both American and British English.
Is Amy in school today?
The students are in school until 4.30.
I'm in school from 8 AM to 3 PM every weekday.
At school
"At school" is used to indicate that someone is engaged in the educational process as a student in British English.
My daughter is still at school. (= My daughter is still a student.)
I spent a lot of time at school studying for my exams.
I was a shy kid at school, but now I’m much more outgoing.
"At school" is used to indicate the place where children go to be educated in both American and British English.
Is Amy at school today?
The students are at school until 4.30.
I'm at school from 8 AM to 3 PM every weekday.
This phrase also refers to being present at the location of the school.
He’s playing football at school.
I’ll meet you at school after class.
Note
In American English, "in school" can refer to being enrolled in any educational program, including college or university. In contrast, British English typically uses "school" to refer to primary and secondary education only, not higher education.
Summary
Being student
American Her kid is still in school.
British Her kid is still at school.
Place
The students are at/in school until 4.30.
Location
He’s playing football at school.