We study grading outcomes associated with professors in an elite university in the United States who were identied ñ using voter registration records from the county where the university is located ñ as either Republicans or Democrats. The evidence suggests that student grades are linked to the political orientation of professors: relative to their Democratic colleagues, Republican professors are associated with a less egal- itarian distribution of grades and with lower grades awarded to Black students relative to Whites.
That is the abstract. The paper, "Partisan Grading," is by Bar and Zussman in the current number of the AEJ: Applied Economics.
An illustrative graph from the paper:
Obviously that is a case study. The paper might suffer from several biases. The conclusions can not be generalized. The findings, however, are interesting.