New Mexico’s highest court began hearing oral arguments Monday for and against the state’s long-running practice of paying for textbooks at private parochial and secular schools.

The six-year court battle was initiated by two parents who say the practice takes money away from public schools and violates a constitutional prohibition on public funding in support for private schools.

The state Public Education Department and private schools say the textbook funding fulfills an obligation to help all students and that the relevant constitutional provision is discriminatory.

The U.S. Supreme Court voided a 2015 state ruling and sent the case back.