RACIST, in my dictionary, is one who spends his or her time promoting and exploiting racism for attention or personal gain. When we start plugging stereotypes into movies merely based on race, religion, ethnicity or sexual orientation, we have become what we despise.
Some of the contributors here remind me of a comment from one of my slower adult students a few years back, "I CAN'T BE RACIST, I'M BLACK!" I think we are all capable of prejudging our neighbor. (...and, yes, Camille, there is also a difference between ignorant and stupid.) If you don't know who, I may be talking about you.
There's a difference between "prejudging" and racism. Prejudging is a thought, sometimes unintentional, while racism is acting on it and even being proud of it. You also need to be in a position of power to be racist, so no, the student who you described as slow was correct, black people cant be racist. They can be prejudiced, but not racist.