In the US, there are 12 grades as you indicate, but above junior high school (grades 6-8), most students prefer to be called high school freshmen, sophomores, juniors and seniors , or 1st, 2nd, 3rd and 4th year students.
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MUSCOVITEWhat are other ways to express the same idea (= "John is an Nth grader")?"John is in third grade."