Question regarding disability

Maybe someone here can answer a few questions, I've looked online and so far haven't come up with much.

My wife works for our local school system in a cafeteria (as anyone who's ever experienced a middle school cafeteria knows they're noisy places). She is totally deaf in one ear and has about 40% hearing in her other ear (this has happened in the last 5 years, she's been at her current job for over 15 years). She has hearing aids which help her tremendously, but she still has problems in a noisy environment.

She is now catching flack from her supervisor for not being able to run the cash register fast enough because of her hearing problems (sometimes she has to ask students to repeat their student number, for instance).

As I understand it her supervisor either needs to move her to another job with equal pay or else arrange for her to retire on disability, is that correct?

Also, if she's moved to a different job is the employer required to give her the same number of hours per week, or can they give her a position with fewer hours (and less money)?

Thanks