How can I say "grade on a curve"?
It is called 标准分. Its opposite is called 原始分.
标准分 is the modified score according to the distribution of the score in the class.
Its calculation method might be different from curved scores, but the logic is the same.
The problem is, in China, modified scores are rarely used. Original scores are used in almost all situations. So the word for the curved score is not wildly used.
标准分 was sort of an education experiment decades ago. Teachers tried but finally decided not to use it. So, people might not understand this word these days. I know this word because my parents who are in their 60s were educators at that time. But I myself never attended any school that uses 标准分 or any kind of modified scores.
And letters grades (优，良，中，差，及格，不及格) are usually only considered as additional information because original scores are almost always available. So usually grades are not very important but the original scores are. The original scores are usually out of 100, 120, or 150.