My guts would say Student/Apprentice or merely Student (your call). I think Apprentice would be OK if they didn't have classes to take as well.
Staff would probably be reserved for full time staff, and since many of the students have jobs with the academy anyways, an apprenticeship would simply be both a form of financial aid and perhaps on-the-job study.
However, that said, I don't think there are any strict guidelines on this, so long as it's easy to figure out what is meant by the Type/Function.