Eight-year-old Sean King says he meant no harm by wearing the make-up, but that didn't stop his principal from calling him into his office. Sean attends Falcon School District 49 in Colorado Springs.
The principal says several teachers and students were offended by the make-up.
The NAACP says the principal did the right thing.
The organization says it's proud of Sean for portraying Dr. King, but says he shouldn't have worn the black make-up.
Sean's mother says she's so upset she might pull her son out of the school.
"I worked very hard, I tried my best. I don't want to be mean to them. It's just a costume. I don't want to insult anybody," Sean said.
Black-face make-up was used by performers in the 1800s to portray racist stereotypes.
No guidelines were ever given for what a student could wear for their project.