Inside Higher Ed just ran a story titled, "Eye of the Beholder," which reports on an article showing that there is a strong correlation between being conservative and not open to changing one's mind and perceiving liberal bias in the classroom.  Similar thing happens with closed-minded liberals — they have the habit of seeing conservative bias.

The study found that students — even in the same classrooms — didn't perceive bias in the same ways (or at all), and those who perceived bias were those who were resistant to changing any of their views. The finding extended to some who identified themselves as being far on the left and resistant to change, and who believed that they had some biased conservative professors. But among both left-leaning and right-leaning students who didn't score high on resistance to new ideas, there was little perception of bias.

In short, if you're dogmatic about your views, you're likely the one to report having a biased professor. (Sidebar: my experience is perfectly consistent with this, as pretty much every person who's ever accused me of classroom bias has either been a blinkered conservative or a raging Marxist.  That said, this, apparently is true of me.)

What explains this variation in perception of bias in the classroom?  The lead researcher, Darren Linvill of Clemson University, proposes:

…[T]here may be elite colleges and universities where students arrive as freshmen used to having their views challenged by teachers, and that might still be "an ideal." But he said that the reality he sees from his research is that this is a foreign concept to many entering college students today.

That's it – challenging a view is a case of bias.  In a way, yes, it is.  It is biased against dogmatism.  (A question: is this a case of self-serving bias, as the dogmatic students tend to do poorly in discussion and blame it on professor bias instead of their own lack of preparation? Is it self-serving to offer that as an explanation?)