The federal government is siding against New York City in a case before the U.S. Supreme Court that parents of children with disabilities are watching closely.

The case is likely to set standards for when localities must reimburse parents for private school tuition for students with a range of disabilities. The New York City Department of Education says it must only pay for private school if, after a child is first placed in a public school special education program, the school is unable to meet the needs of the child. The city claims that any other policy will require it to pay for the bias many parents have toward an expensive private education.

But the U.S. solicitor general, Paul Clement, argues that the city’s policy denies some children immediate access to an appropriate education. In a brief filed to the federal high court this week, the solicitor general’s office claims that the city is responsible for funding a private education for students the school system is unable to serve even when the child has never spent a day in public school.

The case, which will be argued before the U.S. Supreme Court this fall, is the second this year in which the Justice Department has opposed the city. The city won the first case, which involved local property taxes the city had assessed against foreign governments.

A loss before the high court in the current case could cost the Department of Education tens of millions of dollars. During the 2005–06 school year, about half of the 2,240 reimbursement requests the city considered involved children who had never received special education from a public school, according to school officials.

At issue is a federal law, the Individuals with Disabilities Education Act. The solicitor general’s brief argues that the city’s reading of the law creates an unnecessary burden on disabled children by forcing parents to enroll them in public programs that may be ill-suited to a their needs. Under the city’s policy, parents “would have to subject their child to a program they believe would be to their child’s detriment in order to qualify for tuition reimbursement,” the brief argues.

The brief also claims that the New York policy encourages children to be transferred unnecessarily between schools, which “can prove highly disruptive to a child on an educational as well as psychological level.”

The city argues that the city is fielding requests for tuition reimbursement in cases for which the city is capable of offering an appropriate education to a disabled child.

“Public policy does not require that the parents of children who require special education and related services be reimbursed for unilaterally placing those children in private school where the parent’s rejection of the public placement is based upon mere speculation that it is inappropriate,” lawyers for the city wrote in a brief filed in May.

The request for tuition reimbursement at issue in the case, now backed by the Justice Department, was brought by a creator of MTV and former chief executive of Viacom, Thomas Freston. Mr. Freston is seeking reimbursement for tuition for his son, Gilbert, who was diagnosed with attention deficit hyperactivity disorder. Gilbert Freston attended the private Stephen Gaynor School on the Upper West Side.

The appeals chief for the city’s law department, Leonard Koerner, is expected to argue the case for the city. Mr. Freston is represented by the firm Patterson Belknap Webb & Tyler LLP. A lawyer for Mr. Freston did not return a call for comment yesterday.