Like Kay said, they are voted from among the company- you don't have to have any special qualifications. The vote takes place with only AEA members present, on the first day of rehearsal. You can read the description of a deputy's responsibilities
here, in the stage manager's packet. The deputy contacts AEA if they feel any rules are being broken, or if the health and safety of the actors is at risk. They are a representative of the cast, and if an actor has a concern that they don't feel comfortable approaching management about, often they will approach the deputy and the deputy will broach the subject with the SM.
I can't think of a real reason for this in an educational setting, but I have worked on a large show where there was a Principal Deputy and a Chorus Deputy, and the theatre decided to (unofficially) elect a "non-AEA deputy" as well, so that those members of the cast also felt like they had a representative. (Of course that person didn't deal with union issues).
Side note- AEA has recently launched an online deputy support center, with the deputy information available online, and no more filling out hours tracking paperwork. I will be curious to see how the new system works!