Professor Talia Bar from Cornell University and Asaf Zussman from Hebrew University, looked at 3,300 college classes taught by Republican and Democratic professors and found that Democrats seem to be more egalitarian in grading. Republicans tended to give a lot of very high or very low grades, while Democrats have a more even spread. The study, to be published in Applied Economics, was previewed by Inside Higher Ed. From Higher Ed:
Among grades given by Republicans, 6.2 percent were C- or lower, compared to only 4.0 percent of the Democratic grades. But Republicans were also more likely to give out A+ grades (8 percent of their grades, compared to only 3.5 percent from Democrats).
In addition to surveying professors about grade distribution, the study also looked at race. One of the more controversial findings in the study is that while black students overall did worse than their white peers, they were graded more harshly under Republican teachers than Democrats. It's hard to see if that really implies that Republican professors have any more inherent bias than their liberal counterparts, especially since Republicans only made up 10% of the teachers studied. In addition, it looks like the study only surveyed one elite university from 2000 to 2004, so there's no way to know if the results were specific to that school or region, or if they could be applied more widely. Another thing I wonder: would these partisan biases regarding race disappear at a school that had more black professors and/or students? The study only surveyed 11 black professors, and none of them were Republicans: so even though it looked like black professors graded similarly to whites, there weren't enough of them to include it in the study's results.