The US justice department has filed a complaint on behalf of a Muslim girl who was twice sent home from school for wearing a headscarf.
Nashala Hearn was allowed back to school in October
The education authorities said the hijab breached the dress code of the school in Oklahoma.
But the justice department says it amounts to religious discrimination.
The case of 11-year-old Nashala Hearn follows rows in France and elsewhere about whether the school is a suitable place for religious symbols.
Nashala Hearn was suspended twice last year from Benjamin Franklin Science Academy in Muskogee, Oklahoma.
The school allowed her back in October, in her headscarf, but the local authorities stood firm on their dress code policy, which bans all forms of headgear.
The justice department believes banning headscarves violates the equal protection clause of the US Constitution.
"We certainly respect local school systems' authority to set dress standards, and otherwise regulate their students, but such rules cannot come at the cost of constitutional liberties," Assistant Attorney General Alexander Acosta said in a statement.
"Religious discrimination has no place in American schools."
But a lawyer for the Muskogee School District said he was puzzled by the justice department's action.
"There is no federal right to wear religious attire" in
schools, he insisted.
"We are in compliance with federal guidelines from the Department of Education."
The French government has approved a law banning headscarves and other religious symbols in schools, while a state in Germany wants a ban on teachers' hijabs, and secular Turkey has been wrestling with the question for years.