A Jewish student at a Maryland high school was asked to prove that he wore a yarmulke for religious reasons.

Caleb Tanenbaum, 17, was asked by the administration of Northwood High School in Silver Spring to provide a letter from a rabbi explaining why he wore the plain, off-white knitted kippah, Patch in Wheaton, Md., reported.

Caleb, an Israeli by birth, decided recently to wear a kippah, according to the news website.

“All students are allowed to wear headwear according to their designated religion,” Principal Henry Johnson told Patch. “Because our students are not allowed to wear hats and other headgear at school, students are asked for verification when their religious headwear is not traditional headwear that we are accustomed to seeing.”

Johnson said that he has asked other Jewish and Muslim students to provide verification as well.

Rabbi Shlomo Buxbaum, the director of Aish DC, wrote a letter to the school that said, according to Patch, “I ask you, in the spirit of religious acceptance, to allow him to wear his kippah in the school.”

Steven Tanenbaum told Patch that his son was threatened with suspension. The elder Tanenbaum said he complained to the school district, the Anti-Defamation League and the American Civil Liberties Union. 

He is calling for a letter of apology and reform in the headgear policy.