Washington — Public-school students are performing better on most states’ reading and mathematics tests than they did in 2002, when the No Child Left Behind Act was passed, according to a report released today by the Center on Education Policy, a public-school advocacy group based here.

The report cautions, however, that it is impossible to know whether the No Child Left Behind Act is responsible for the enhanced student performance — because many states and school districts have sought to make improvements on their own and because no public schools are exempt from the law to serve as points of comparison.

The center’s analysis found much more evidence of improvement at the elementary- and middle-school levels than at the high-school level. In most states that provided data on low-income students or various racial and ethnic group, the report says that achievement gaps have narrowed. —Peter Schmidt