Disability advocates are calling out a group of Republican senators for proposing changes to federal education law that they say would lower expectations for students with disabilities.

In a letter sent to six Republican senators last week, more than three dozen disability advocacy organizations asked the lawmakers to reconsider their proposal to reauthorize the nation’s primary education law known as No Child Left Behind.

The senators introduced a bill in September that would allow students with “the most significant cognitive disabilities” to be held to different academic standards and take alternate achievement tests. The bill does not specify any limit on the number of students who could take the modified exams.

Disability advocates say they are concerned that the approach could jeopardize accountability by allowing an excessive number of students to take alternate tests.

“If large numbers or possibly all students with disabilities are given alternate or modified assessments, we will effectively — and under the proposed language — legally create a separate education system for students with disabilities,” the disability groups, including The Arc, Easter Seals and the Council for Exceptional Children, wrote in a joint letter.

Under the current No Child Left Behind law, only a small percentage of test takers are allowed to use modified exams.

U.S. Secretary of Education Arne Duncan told Congress in March that including most special education students in testing was one of the law’s benefits.

“NCLB was right to shine a bright light on achievement gaps and set a clear expectation that all students must learn to the same standards,” he said. “This has led to great progress in schools focusing more on the needs of English learners and students with disabilities and other at-risk students.”

Nonetheless, some have criticized No Child Left Behind saying that holding students with disabilities to the same level academically as other students is unrealistic.

It’s unclear whether or not the Republican proposal will gain much traction. The U.S. Senate Health, Education, Labor and Pensions Committee plans to consider reauthorization of No Child Left Behind next week, but it’s expected that a separate bipartisan effort may take priority at that time.