The Bush administration bowed to intense pressure from local school chiefs this week and relaxed requirements that severely mentally handicapped students sit the same exams as other students.
But 90 per cent of pupils with special educational needs will still have to take standardised tests.
Holding pupils with special needs - ranging from mild dyslexia to autism, emotional disturbance or profound retardation - to the same standards as their regular peers, emerged as a major sticking point in the White House's drive to roll out a stringent academic accountability regime across US schools.
Inadequate standardised test-scoring by SEN students was the biggest common denominator of schools branded as failing in performance league tables compiled recently under the sweeping education reform.
The No Child Left Behind Act holds schools accountable for performance across a range of subgroups of pupils, including the historically-neglected SEN students. Bush's officials defended the exacting special education requirements - part of the Act's mandate that 99 per cent of students demonstrate reading and maths proficiency by 2014 - as necessary to eradicate slack teaching.
But incensed education chiefs said they are being asked to accomplish the impossible and that the law is unfairly stigmatising perfectly good schools.
"In New Hampshire the vast majority of schools are considered failing, because SEN students scored low," said Jean Parsons, special services co-ordinator of the state's Plymouth school district.
Special needs pupils' performance also lowered the results of most failing schools in South Carolina, Pennsylvania, New Jersey and Georgia.
"I've heard special needs students saying they feel responsible for the whole school failing," said Parsons.
On Tuesday, Bush eased some pressure by granting additional dispensations to pupils diagnosed with "significant cognitive disabilities" such as Down's syndrome, traumatic brain injury, severe learning disabilities and those who have trouble holding a pencil. This raised the proportion of SEN students excused to pursue their own pace of study from 1 to 9 per cent.
"It strikes a balance between being realistic about assessing students with disabilities and protecting the rights of students and parents on accountability," said an education department spokesman.
Cathie White, a special education teacher at Pennsylvania's G.A. Stetson middle school, was unappeased. "It's a start, but you're still dealing with all these kids who are learning disabled."
However, James Wendorf of Washington DC advocacy group, the National Center for Learning Disabilities, said some schools were too quick to complain.
"The vast majority of students with learning disabilities should be included in the test schedule. The problem is they haven't been held to high standards - raising expectations is critical."
Nine per cent of America's pupils have special needs, according to the education department.
On top of special education, schools must meet roughly 60 performance targets under No Child Left Behind. A slip-up in just one is enough to cast them on to publicly disseminated lists of schools failing to show "adequate yearly progress". If they fall short two years running, escalating sanctions kick in, culminating in the firing of staff and closure unless exam marks improve.