Details are here. An excerpt:
"[. . .] A forthcoming study finds that there may be notable differences. Democratic professors appear to be "more egalitarian" than their Republican counterparts when it comes to grading, meaning that more of the Democratic grades are in the middle. Republicans are more likely than Democrats to award very high grades and very low grades.
Another key difference is that black students tend to fare better with Democrats than with Republicans. [. . .]"