This pisses me off when people paint any use of the word black as racist.
Say you claim, "White skin, because dark skinned people were designed by satan" is racist. But that is not the case, so you are not racist. Racism is about hatred of other races, not noting certain trends or using a certain word. "That person is Black" is not racist. It's an easy way of determining between people and after hair colour is the most easy thing to distinguish a person by.
I think Eddie Murphy said it best, "Black people have big lips. It's not racist, it's fact" in his stand up show show "Raw". Just like white dudes have no rhythm (also completely true).
What really drives up the wall is when I chose Pokemon White over black (Don't judge me. Pokemon red came out when I was young and naive and stands as the pinnacle of my childhood) and a friend claimed "that's a bit racist isn't it?"
NO! It's racist when there is genuine hate behind it, and my decision was regarding the fact that White version features a black robot dragon with glowing red eyes that breathes lightning.
Half the time the term comes from people who are so desperate not to be painted as racist that they blow the term out of context. If a black kid had written "prefers dark skin" I doubt the teacher would have battered an eye. And so the matter does become racist towards white kids, who aren't allowed to use certain words because they are white.
Bottom line, an arsehole is an arsehole, regardless of colour, gender, ethnicity, sexual orientation or beliefs and your teacher is a bigoted jerk for claiming otherwise.
Sorry about the rant. I blame the the booze.