"제 남동생은 초등학생입니다."
Translation:My little brother is a primary school student.
The answer given was 'grade school student'. This is nonsensical. What is a grade school student? The answer should have been 'primary' or 'elementary'. I'm an English teacher and I don't understand this.
Grade school is a typically North American expression meaning elementary school. It is not nonsensical, it is just slightly colloquial and a little outdated.