IT MUST BE nice for the administration's education team to hear, for once, praise from teachers unions and other long-standing opponents of the president's No Child Left Behind law, which mandates annual assessments of both students and schools. That's what it got last week when Education Secretary Margaret Spellings announced that states could apply to meet the requirements of the law by using a new set of rules, known as "growth models," that would measure student "improvement" instead of straightforward student "proficiency" as in the past. National Education Association President Reg Weaver enthusiastically supported the change, on the grounds that such a system will "truly reflect the great progress we are making in the classroom."

The trouble is, there isn't any evidence that American schools or American teachers are making great progress in the classroom. On the contrary, the results of recent standardized tests show that on average American children are making very little progress. There are exceptions -- but mostly in the states, districts and schools that have been using regular assessments and accountability standards the longest. Four years after its enactment, the No Child Left Behind law is no slam-dunk success, but the basic principles around which it was organized -- accountability, assessment, standards -- have not been disproved.

Implemented carefully, a "growth model" that measures annual improvement could spare schools that are making genuine progress, and that are on track to meet the ultimate goal of 100 percent proficiency, from being wrongly labeled as failures. The administration insists that it will require any state using this model to track students individually, and to meet the same long-term standards as others. But there are risks: If improvement becomes a substitute for achievement -- and if some schools are allowed, indefinitely, to escape meeting standards of proficiency -- then the nation will be reinforcing the two-tiered school system, with different standards for low-income and minority children, that the reforms were intended to dismantle. Ms. Spellings is right to treat this as a pilot project, not a permanent change.