The exam grader must evaluate the answer sheets fairly.
彼女はTOEFLの採点者として働いている。
She works as a scorer for the TOEFL test.
評価者 is a more general 'evaluator' or 'assessor', not limited to test grading.
Compound of 採点 (marking, scoring) + 者 (person). The exact historical derivation is straightforward; the word follows a common pattern for agent nouns.