If you can't trust a professor to professionally and impartially grade someone's work, the system would likely collapse. This is not to say that there hasn't been cases where professors have been shown to be biased, there has. But the premise of universities is to give professors some autonomy in the way they teach and evaluate students.
No system is 100% the question is are we good enough. As a white male I haven't seen many problems - but also because I'm in the group least likely to see one.
