The irrationality is assuming that you are able to be an impartial observer who can 'regularly' notice 'Chinese students sitting in the back of class openly whispering during exams' and that means that Chinese people cheat more.

We have so much evidence that these sorts of observations can so often be wrong. You are likely to not notice all the majority group people doing this, or at least not connect it as a pattern. Once you have an expectation that Chinese people are cheating, you are more likely to see innocent behavior as cheating, or even if they ARE cheating, more likely to catch them than the white guy sitting next to you also cheating (because you are paying more attention to the person you EXPECT to be cheating)

Now, of course, this could be wrong and your observations could be right. The irrationality comes in assuming you are right just because you observe something.