Plural of grader; people who assess and assign grades to work, or students in a particular grade level.
From 'grade' plus agent suffix '-er'. 'Grade' comes from Latin gradus 'step, degree'. The educational sense developed in American English in the 19th century as formal schooling became systematized with year-based progression.
The term 'graders' reflects the industrial organization of education that emerged in the 19th century - students became products moving through an assembly line of learning. Interestingly, some education systems are now moving away from rigid grade levels toward competency-based progression, potentially making this word obsolete.
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