The US Supreme Court said it will decide which bathrooms should be used by transgender people, a highly sensitive question with national political resonance.
The case involves 17-year-old Gavin Grimm, who was born a female but identifies as a male. Grimm filed suit to be allowed to use the boys' bathroom at his high school in Gloucester County, Virginia.
Arguments in the politically charged question will be heard between now and late June, making it easily one of the highest-profile cases set for the court's current term.
The Obama administration has said public schools should grant access to toilets and locker rooms based on the gender with which a student identifies, not the birth gender.
Federal authorities, seeking to fight discrimination, have threatened school systems with a loss of federal funds if they fail to comply.
But more than a dozen Republican-controlled states, bitterly opposed to that approach, are challenging the federal government in court.
Some conservatives see the directive from Washington as improper interference in local school affairs and an abuse of executive power.
The defenders of this position point both to their religious values and questions of security and privacy. "No men in women's bathrooms" is the slogan they repeat on social networks and in rallies.
Leading the opposition, North Carolina adopted a law requiring that transgender people use toilets corresponding to the gender on their birth certificates.
That law, affecting a tiny minority of the population -- less than 1 percent, according to some studies -- has been condemned as discriminatory by leading figures from civil society, from the business and sports worlds and from the Democratic Party.
Since the death in February of Justice Antonin Scalia, the Supreme Court has been operating with eight justices, instead of the normal nine.
With four conservative and four liberal justices, the chances of tie votes -- and thus of weakly-worded decisions -- become greater, especially on such ideologically charged topics as this.
One of the first tasks of the next US president will be to nominate a ninth justice -- unless the Senate moves quickly after the election and before a new Congress is seated to confirm President Barack Obama's nominee, Merrick Garland.
"It is a bit disconcerting that the Supreme Court would agree to review this case with only eight members on the court," said Elizabeth Cooper, a professor at Fordham Law School in New York.
"One can only hope that by the time the case reaches the justices, they will be at their full complement of nine."