I take it "klas" can only mean "grade" as in the Americanism "third grade" (nth grade), meaning year 3 (n) of school? (And not in the sense of "mark")
Just to be different, in recent years, we Brits now refer to "year n" in the same sense as the US "nth grade", where formerly different usages aplied to different levels of school. So if the meaning is strictly "grade" the british usage is now "year". Year 6 is first year of secondary age ten to 11.
Could it not be "I am not sitting in this class"? I guess that does sound a bit clumsy, but it has a slightly different meaning.
You don't "sit" in a year grade, even if you do in dutch. Klas here isn't a classroom.