He was stopped just short of walking the stage for his diploma, and was told he wouldn't be allowed to do so unless he switched his cap and took off an item around his neck.
The 17-year-old had placed his Native American regalia on his graduation cap. He refused to switch it out or take it off.
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"It's not just, like, a decorative thing," he said. "It's a religious belief to hold these feathers sacred."
By refusing to give over the sacred feather, he instead gave up his opportunity to take part in his graduation ceremony. He watched from the stands with his family, including his mom and sister.
"It's awful, and I am so proud," said Megan Bang, mother.
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"He's always been a humongous role model for me. He's paved the way for change," said Miigis Curley, sister and fellow Evanston Township High School student.
The Evanston Township principal paid a visit to his home Monday morning, delivering his diploma in person.
"He apologized for the situation, and I appreciate that," Curley said.
Despite telling graduating students additional apparel or adornment is not allowed, in a statement school officials said they are "reviewing the graduation guidelines, particularly as they relate to acknowledging the history and stories of indigenous students."
While the family waits for official changes to be made, Nimkii said he'll be going to the University of Wisconsin-Madison to study Environmental Engineering.