LANSING, Mich. — A federal appeals court this week revived a challenge to the funding of the No Child Left Behind education law.

School districts in three states and the nation's largest teachers' union sued the government, arguing that schools should not have to comply with requirements that aren't funded by the federal government and that the government is imposing unfunded mandates, even though the act itself prohibits unfunded mandates.

Plaintiffs include the Pontiac, Mich., school district and eight districts in Texas and Vermont, along with National Education Association affiliates in several states.

The NEA is paying for the appeal.

Chief U.S. District Judge Bernard Friedman in Detroit dismissed the lawsuit in November 2005, saying the plaintiffs failed to support their claim, but his ruling was reversed by a three-judge panel of the 6th U.S. Circuit Court of Appeals in Cincinnati, in a 2-1 decision released Monday.

The court majority said the law fails to provide clear notice about who must pay the additional costs of compliance.

U.S. Education Secretary Margaret Spellings said No Child Left Behind is a compact between the states and the federal government, not an unfunded mandate.

"This decision could undermine efforts to improve the education of our nation's children, in particular those students most in need," Spellings said in a statement.

The case would be sent back to federal district court unless the ruling is appealed. Spellings said the government would explore all legal options.

Reg Weaver, president of the NEA, said the federal government should provide more money for schools to comply with No Child Left Behind.

"It's a matter of asking the federal government to live up to this ruling," Weaver said.

The No Child Left Behind Act, backed by President Bush, became law in 2002.