From the informal discussion letter:
Under the ADA, a qualification standard, test, or other selection criterion, such as a high school diploma requirement, that screens out an individual or a class of individuals on the basis of a disability must be job related for the position in question and consistent with business necessity. A qualification standard is job related and consistent with business necessity if it accurately measures the ability to perform the job’s essential functions (i.e. its fundamental duties). Even where a challenged qualification standard, test, or other selection criterion is job related and consistent with business necessity, if it screens out an individual on the basis of disability, an employer must also demonstrate that the standard or criterion cannot be met, and the job cannot be performed, with a reasonable accommodation. See 42 U.S.C. § 12112(b)(6); 29 C.F.R. §§ 1630.10, 1630.15(b) and (c); 29 C.F.R. pt. 1630, app §§ 1630.10, 1630.15(b) and (c).
Thus, if an employer adopts a high school diploma requirement for a job, and that requirement “screens out” an individual who is unable to graduate because of a learning disability that meets the ADA’s definition of “disability,” the employer may not apply the standard unless it can demonstrate that the diploma requirement is job related and consistent with business necessity. The employer will not be able to make this showing, for example, if the functions in question can easily be performed by someone who does not have a diploma.
However, there will obviously be a large pool of jobs that meet the "consistent with business necessity" requirement such as jobs as doctors, pharmacists, nurses, etc.