The changes have already resulted in the dismissal of more than 500 cases filed by a Michigan-based disability rights advocate, Marcie Lipsitt, who had opened more than 2,400 web-accessibility complaints against various educational institutions in the last two years.

Her complaints resulted in more than 1,000 resolution agreements with institutions such as colleges and universities, which vowed to make their websites accessible to people who are deaf or blind or who struggle with fine motor skills. The department has started revising some of those agreements in light of the new case-processing manual, Ms. Lipsitt said.

Last week, House and Senate Democrats wrote to Ms. DeVos denouncing the manual’s changes. They noted that “frequent fliers” are usually advocacy groups or individual advocates who have either expertise or resources that allow them to identify systematic violations.

“Selectively choosing which students will be protected and which complaints will be investigated is a violation of the law and will result in serious civil rights violations going unchallenged,” said the letter, organized by Senator Bob Casey of Pennsylvania.

For the last decade, the National Federation of the Blind has helped its members navigate technology in schools and on college campuses, said Mark Riccobono, the organization’s president. But progress has been slow, particularly in higher education, where the organization has seen members drop out of school because they could not gain access to the registration system or classes that were available only online.

“This is going to have an effect on the class of blind people who really need the Department of Education’s Office for Civil Rights to continue to enforce the avenues of access that really aren’t being met,” he said.

In 2009, the organization filed several complaints with the civil rights office and the Justice Department against colleges and universities that were deploying inaccessible Amazon Kindle devices in classes. The complaints resulted in resolution agreements with several colleges and universities.