Passed by the U.S. Congress in 1990, the ADA guarantees equal employment opportunities for persons with disabilities. The ADA also provides for reasonable accommodations to increase the numbers of students with disabilities in primary, secondary, and postsecondary education.
Reasonable accommodations are provisions that assist students in performing at their best. In the school setting, reasonable accommodations might include preferential seat placement, signers in the classroom, or changes in a test format, including extended time.
For further explanation:
Wrightslaw provides a page on "Section 504, the Americans with Disabilities Act, and Education Reform".