Should also accept "good grade" as being correct.
成績表에 좋은 點數가 있어요.
Maybe this means something like term GPA? Otherwise, yeah it would be plural and grades in the US.
Why wouldn't it accept "good mark"?
Can a school mark be numeric?
In UK English, yes. I had a British math professor who always used "mark" or "marks" exactly how we would use "score" or "points."
Better be more than one.
In South Africa, we usually just refer to it as a "report".
"The report card has good grades."는 어때요?