How do you say to grade a paper in french?

Most directly translated:
« La Huitième Année » aka: The 8th grade in the "North American" English system.

In Québec it would be: « Secondaire deux » aka: secondary two, they start high school in grade 7, and call it secondary (school grade) 1, grade eight is secondary 2, etc.

In France it would be: « Quatrième » aka: 4th, they count high school grades backwards, Sixth, Fifth, Fourth, Third, First, Final.

The only "number" that is the same in all three systems is grade 6.