I grade my university students' work with Herbin Violette Pensée ink and a Platinum Plaisir fountain pen. The symmetry of using a student's ink to grade students' work tickles me.

For other people who grade big stacks of papers, nota bene: fountain pens with a soft nib are a lifesaver! They require almost no writing pressure, which is so much more comfortable. You also get to use fun ink colors.