"We aren't race-baiting — we're illustrating what happens when black history is taught once a year," says Illinois college student Emmanuel Imarenezor, 19.
"Every day this is the type of microaggressions that black faculty, parents and students have to face in this racist system," wrote an activist.
Footage shot by a student shows a math teacher getting emotional as she removed the door decoration, which showed the athlete and activist taking a knee.
"I think it was teaching children it's ok to bully," Alabama mother Amanda Branco told local news station WBRC.
"I'm angry as hell that my daughter was not allowed to represent Black History Month," said the tearful New Jersey mom.
The expected task is to research and present an African-American historical figure, and students are encouraged to wear costumes depicting the icon.
"It's really insulting," said a local NAACP official about a Virginia elementary school that had students participate in an activity in which they pretended to be slaves along the Underground Railroad.
The school says that it was an opportunity to learn about the Great Depression — not about slavery.