For the first time US schools will award students credit for progress even if they miss testing goals, in a pilot scheme announced by the Bush administration.
From next February, 10 states will experiment with the so-called "growth model", under which individual students' performance is tracked throughout their school career to measure improvement.
The results of the trial will be reviewed at the end of the school year to decide whether the method should be incorporated into the No Child Left Behind Act, the White House's sweeping education law.
The trial could herald a major overhaul of academic accountability regime schools face under the Act, which prescribes sanctions for those missing targets, culminating in staff dismissals. "It's potentially a huge shift in policy," said Andrew Porter, education professor at Vanderbilt university.
The question of refining school performance measures has assumed growing importance for policy-makers and education chiefs as US school reforms move forward, Professor Porter said.
Currently, schools must meet annual reading and maths targets, in which a given percentage of students in several socioeconomic and ethnic minority groups must reach the level expected of their age - towards an ultimate goal of universal proficiency by 2014.
Critics have complained that students might record impressive testing gains but still miss targets, and that grading schools on student improvement offers a fairer reflection. But supporters contend that budging even an inch on holding schools to absolute standards represents a slippery slope.
US education secretary Margaret Spellings denied watering down standards.
"There is nothing inconsistent between this pilot and the law," she said.
Professor Porter said officials could face a tough juggling act between growth-based accountability approaches and striving for universal proficiency within nine years, but that trying to fine-tune the system was healthy.