To give someone a low grade or score on something. A noun or pronoun can be used between "grade" and "down." I'll grade down anyone who hands in a sloppy book report.I hope the teacher grades Joey down for talking during my presentation.
grade someone down (on something)
to give someone a low ranking, rating, or score on some performance. I had to grade you down on your essay because of your spelling.Please don't grade me down for a minor mistake.
grade down
v.
To give someone a lower rank or score, usually with respect to something evaluated: The teacher graded me down on my English test because of my terrible penmanship. The driving instructor graded down our group because we weren't listening.